How would I install a switch to turn my neck pup on? I have a HSS strat with a master tone and volume.
I'm not sure why you would want a switch for the neck pup as you should already have a 5-way switch, but you could use a SPDT switch, put the pickup's hot output to the middle lug, then (depending if you want the switch up = on, or switch down = on), solder the wire to the 5- way switch where the hot out. used to go to the top or bottom lug, then attach the remaining lug to a ground point
Sorry. Ya I have a 5 way switch.

But with a switch for the neck I could get neck and bridge combo.
oh, well then the switch I said won't work...
If you wanted to buy a new pickguard, you'd be able to make 3 small switches in place of the 5-way, each an on/off for each pickup.
If you don't want that, then you could do the same switch that I described in my earlier post, except instead of having one position grounded, have it go straight to the volume knob (bypassing the 5-way)