I'll save everyone the long story of why this has to be done, but I need to leave my guitar in the trunk of my car overnight. It's about 78 degrees outside with high humidity. Everything I'm reading says this is an absolutely horrible idea.

However, every guitar that got to every guitar store in the country was brought there by truck or plane which was not climate controlled. Some times they take a trip for several days, in the middle of summer (oh the horror!). In theory, I understand all the reasons it is a bad idea. But seriously, is anything bad actually going to happen?
You didn't say if it's electric or acoustic. It's a bad idea for acoustic because heat can soften the glue and you'll end up with a six string bow.
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Electric will be more tolerant but some didn't survive the shippings.
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I once left one of my Squier strats in the trunk of my car as it was shipped across the country (~five days or so), and it arrived just fine. That said, the only reason I even considered it was because it was a Squier.

Regardless, I've found that it's not the trunk you have to worry about, usually; it's the front and back seats because the sun will shine in through the windows and superheat the air inside. Even so, I was able to bring my dad's Telecaster home in that same car, while it was in the back seat with the sun directly on it because (I assume) it was in a black case, which did a lot to absorb the heat.

Unless you live in the desert, nighttime will probably be cooler than daytime, so you should be okay. It's still not something I'd risk for a guitar I cared about. At least make sure it's in a case.
It's really important to note that not everyone lives in climate controlled areas or areas with mild temperatures. This idea of keeping your guitar at x temperature is pretty ridiculous when guitars exist worldwide, it's about limiting extremes.

I live in a part of the world which very frequently goes above 100 degrees with humidity near 100%. Pretty much during the wet season the temperature won't go below 78. I have 3 guitars that 'live' there with me and they are all fine. I realise it's not the healthiest of environments for guitars, but I (and a huge number of guitarists globally) have no choice and no real climate control.

Furthermore, I've had to leave guitars in my car as well. Not for extended periods, and I avoided the sun, but again they were absolutely fine.

With that in mind, the temperature inside your car will be fine, PROVIDED it's not parked in the sun. If it's left in the sun it's basically going to turn into an oven. The ambient temperate of 78 degrees won't be an issue, the issue will be the heat from the sun. Park your car in the shade/undercover and crack a couple of windows if you have to.

Definitely something to avoid if possible, but it's highly improbable you're going to do any damage. The only time I've heard of someone's guitar breaking from heat alone was when they left it in tray of their ute under a black cover, parked in the sun.
Quote by shiggityswah
This idea of keeping your guitar at x temperature is pretty ridiculous when guitars exist worldwide, it's about limiting extremes.

It's not ridiculous. It's just not mandatory. The rest of your post even admits that it's optimal.

If you have to leave the guitar in the trunk, an electric should be fine overnight, I wouldn't worry about it. But the point is, if you can avoid it, you probably should.
Thanks for all the input. I'm not going to worry about it. I should be able to remove it before it gets too hot tomorrow.
Here in sunny CA, car interiors routinely get up to 160 degrees. That's hot enough to soften some glues. Back in Iowa, where I grew up, we had that problem and the opposite of the spectrum as well; in the winter temps dropped to well below 0 F.

The trick, when you're confronted with extremes of temperature or humidity, is to keep the guitar from experiencing warming or cooling from these extremes rapidly. Keeping the guitar in its case, and even wrapping the case in a packing blanket or quilt, will slow down the changes. Let the guitar acclimate for a while before opening the case -- several hours if you can.

Probably best to protect the guitar from movement during these periods as well.

Friend of mine had a very pretty '68 Gold Top. He'd met a lady after a gig and went home with her afterward. It was around 10 below when he finally got home and started unloading his gear. He dropped the LP case on its butt end, panicked and opened it up to see if the guitar had survived. It had, but there was a spider work of cracks in the nitrocellulose lacquer radiating from the bottom strap button up the front of the guitar. He slammed the case closed, took it inside and, through some twisted logic, decided that if he warmed the guitar up, maybe those cracks would disappear. Sure enough, when he opened the case, they were gone.

Except, of course, that they weren't. They'd just tightened up enough that they were tough to see. A few months later, they reappeared, as dirt and moisture (sweat) began to work its way into the cracks.
In addition to keeping the guitar in a case, loosen the strings until they’re slack, or take them off entirely. If the glue softens a little with no tension in a case the neck isn’t likely to move much.
These guys are right on when they point out that you want to mostly avoid the RAPID changes in temp or humidity. Wrapping blankets around a guitar case seems to make good sense. The absolute worst situation to avoid is a rapid drying out of the instrument. In an extreme case, the wood itself will be splitting, never mind the glue. Always try to plan your comings & goings in a way that helps the guitar to SLOWLY acclimate from one environment to another (temperature & humidity-wise) by keeping it in the case & letting it gradually warm up or cool off. If it has been in a hot car a while, and you bring it in to an air-conditioned building, allow it to remain in the case at least a couple hours to let it cool off before taking it out. However, you may occasionally open & shut the case a few times to "fan" the guitar and help the process along. You might want to carry a few soft, absorbent cloths to wipe down a guitar that is doing a good imitation of a dew collector. The same goes for a guitar that spent the night in a cold vehicle. keep it in the case as long as possible to allow it plenty of time to adjust to the new environment. Overnight is usually sufficient. If you have to leave right now to go to a gig, and it's either hot or cold outside, bring a blanket & leave the guitar to be carried out to the car only when you are ready to drive off, and then bring it out and stow it in the car with the blanket wrapped around it (around the case, that is) and make the trip. The back seat is a good place to use, because you will at least have an idea of how hot or cold it is while you are driving, and you can make use of the heater or A/C to maintain a moderate climate in the car. If you feel the need, pre-heat or pre-cool the car interior before bringing the guitar out to the car. It's never a good idea to allow a guitar to get to an extreme temp in the first place, of course, but gigging life has a way of punishing us for implementing our tools of the trade on demand, and things can happen. Maybe the guys didn't leave you any room in a vehicle interior for your guitar, and you have to shoehorn it on top of the amps in the back of a trailer, and you'll barely have time to set up before you play. Make sure to at least get the guitar inside immediately upon arrival and give it as much time in the case, indoors, as possible before opening it. You don't want to pull a cold instrument out of the case and blast it with an array of hot stage lights. There's an example of a rapid change in temperature, the thing you want to avoid. I haven't heard of any one who has conducted any testing or experimentation, and I suppose no one is willing to wreck an instrument to find out the facts. But I did read a good story of how Dan Erlewine at Stew Mac re-built Albert King's solidbody after it was in a flood, and over time the glue joints crept all over. That was a major reconstruction/restoration! That guitar was at least allowed to dry out very gradually, so there was no actual splitting of the woods. If you have dew on a guitar, or a spill for that matter, be aware that the END GRAIN of the wood is the most vulnerable to moisture entry, and I mean it can soak up water like a sponge and swell itself to death in almost no time, then dry out too fast and drive the nail on the coffin when it splits into sections! Keep wiping it down if you have to play it under those conditions, and hope for the best. You may be wise to evaluate the reasons why you think the guitar is "safer" in the trunk overnight, if there's a likelihood that this will end up ruining it eventually. Many non-players have no idea how to care for a guitar, and they may minimize or even outright sneer at your attempts to preserve your guitar. Just remember you don't owe it to them to wreck your instrument for the sake of their ignorance, and you have possibly a lifetime of history with your treasure. FWIW, I have never thought much of the huge variety of guitar humidifiers. If you create an isolated climate for your guitar inside the case, then you are almost guaranteeing that you'll be subjecting it to rapid changes in humidity every time you take it out or put it away. Just my opinion. Thanks for reading , and I hope I've been helpful in your quest to preserve your guitar.
You're guitar should only be in your case indoors.
What happens to a dog when you leave it in a hot car for a while?