F150 electric windows not working

This is for a friend of mine. He has a 1997 F150 4WD XLT pickup truck with a 4.6L engine.
The problem he is having is that both electric windows stopped working at
the same time.
Here is some additional information in case it helps. Before this happened, he was having a problem with the overhead light in the cab. To get the light to come on, he would need to push up on the ceiling near the light, so obviously there was some type of loose connection there. Now, I'm pretty sure he said that neither the electric windows nor the overhead light work at all.
He did try checking and replacing/bypassing all of the fuses but that didn't work.
Shortly before the electric windows stopped working, he had to replace the entire rear differential and axle. After that, everything was working. But about a day later he heard a noise in the universal joint and he had to drop the entire drive shaft and replace the rear universal joint. Immediately after putting the drive shaft and universal back together, the electric windows stopped working. I don't see how those two things could be related, but that is the sequence of events.
He has the door panels off, and he was able to get some print-outs of the wiring diagram and what looks like the locations for various relays, switches, and a "GED"(?) or "GEM"(?) controller.
Now, after all of that explanation and background, here are the questions:
Ordinarily, how hard would something like this be for an experienced repair shop to diagnose and locate where the problem may be located and determine what need to be replaced or done to fix this? I know that electrical problems can be difficult to locate, but is there a fairly simple process that a shop can go through to locate the exact source of the problem the works in MOST cases? He doesn't have much money, so he can't afford to pay for hours and hours of time trying to locate the problem.
Or, are there some things that he could do or check or try on his own to try to isolate where the problem is? For example, if he can find where some of the relays are located, can he try doing some type of jumper across each one to check to see if that solves the problem?
He could also go to a junk yard and remove and buy various relays, switches, and the GED/GEM? etc., if that might be worth trying.
Thanks for any ideas or suggestions.
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That's too much of a coincidence.

I wouldn't jump things. Use a volt/ohm meter to diagnose the problem using the correct wiring diagram. As for how much it would cost or how much trouble it would be to figure this out depends on where the problem is and how fast the tech can work on it. Most electrical problems are a mystery to mechanics. The turn signal fuses on my Porsche kept blowing and it was traced to the backup switch on the transmission (those silly Germans). It took the mechanic forever to trace this and I got charged the standard hourly rate.
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My first "guess" (and this would be a GUESS) would be the wiring in the rubber convolute at the drivers door. After 13 or 14 years of opening and closing, the wire gets brittle from the constant flexing and PFFFTTT!
Proceeding with amateur attempts at "repair" is a fast way to create more grief than is necessary. As you have seen, electrical is hard on most of those that are faced with with it... proper volt drop testing is beyond even many experienced techs.
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Thanks. I'll pass that on, especially about the rubber convolute at the driver side door.
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Jim Warman wrote:

This type of testing is within the scope of some electrical engineers and technicians. You have the wiring diagram; the most difficult part will likely be the identification of certain components, accessing them and knowing how the harness is routed.
One poster who mentioned a volt-ohmmeter; that would be the correct tool to sort this out. They are almost throw-away cheap nowadays.
Maybe there is a local trade school or radio amateur radio club (do they have those anymore?) who can suggest a candidate.
Compared to troubleshooting just about any modern piece of electronics, this is pretty simple.
Good luck,
Ken
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