98 F150 8-4.6 will not even try to go into 4wd. There is no power going to the switch in the dash. Fuse is good, but the truck has a history of water leaks from the windshield. My question is, does the switch get it's power from the GEM, and is it likely the GEM has been ruined by the water, or can it possibly be dried out? Or am I missing somethinf in the wiring diagram, and the switch gets power someplace else?

 

Your all backwards in thinking.
The dash switch only signals the GEM module to request it to act and effect 4H and 4L operation.
It's very likely not the switch.
If the GEM has been getting soaked from a leaking windshield it likely is the issue.
You may have other things that don't work as a result such as the wipers, interior auto lamp off etc.
4X4 is a series of actions that must complete in a chain to get the front axles to lock up.
First action is the transfer case motor is commanded by the GEM to rotate to it's first position.
If not, it could be the GEM, a fuse, a relay or the motor.
Start with this to see what is not happening.
Good luck.

 

Well, I was pretty sure the switch wasn't the cause, just a result. Is there a way to "reboot" the GEM, or does it just need to be replaced. I have it inside, with a heater running, trying to get it dried out, and defrosted since it's below zero here this week. I've tried doing a battery disconnect, but that didn't seem to help any.

 

The GEM is not able to reboot, just programmable firmware sections that respond to inputs and produce output control functions..
4X4, wiper control, power windows, auto interior functions are all separate control functions..
Good luck.

 

Well when I priced this guy on replacing the GEM, he said he'd just go trade it in. I just heard he got a wholloping $500 for it towards a new Tacoma. Oh well, my truck runs fine.