I have a MIM Deluxe players strat, and the only issue I have is that I wish there was a 22nd fret. Does anybody know if an American strat neck with 22 frets would fit on my mexican?
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Don't really know, probably should though. Same parts, just assembled somewhere else.

You'd have to take some careful measurements though, especially with the addition of another fret. The distance from the nut to the 12th fret has to be the same as the distance from 12th fret to bridge saddles, or it will not intonate right. If you swap necks to one with one more fret, it's probably longer and that will change the scale length so those two measurements are not the same any more. If you add one fret, that means the bridge to 12th fret measurement will be about 3/8 inch longer than it should be, no matter what you do it probably will not get good intonation. If the neck is longer...

Will it fit the neck pocket? Take some measurements. Anywhere you buy a neck from they should be able to give you pretty accurate measurements so you can measure your neck pocket and see if it will fit. No need to take the neck off, just measure the neck in place. (neck not pocket) That's the same size neck you need.
Hmmm...I wonder what this button does...
I know the string spacing on American Strats is different (might just be at the bridge anyway), but I believe MIA and MIM necks are interchangeable. The real issue is whether the additional fret will be an issue. I'm not too sure about that. It might depend on the neck.
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It will fit. The extra fret just hangs over the pickguard.

The new 2015 deluxe strats have a different heel which would require some modification. Standard strat necks and/or ones from earlier years will fit just fine unless it's a really old 3-bolt neck.
It will fit. The extra fret just hangs over the pickguard.
Scale length is unchanged.

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it will fit, but you may have to cut away a little bit of the wood from the heel of the 22 fret neck. Line up the 21st fret of both necks and you will see if any wood needs to be removed