2000 379 fuse problem

Sounds like more than just a fuse box issue, hope it is cheap because it can be a nightmare to fix.

Grounds or lack of can also be a major problem.

 

Did you check the big relay behind the dash/tach? If the cab leaks that relay sits under the firewall and wiper linkage just enough to fill with water and rust out. I'd check that first

 

I'd walk away and find something that doesn't have $5k worth of known issues. A 24 year old truck will usually have a lot more problems than that.

 

I would RUN away as fast as possible.
Leaking cab marker lights and windshield drain right into major electrical behind dash.
I will bet the fuse box back side is corroded and needs complete replacement $$

 

Likely the accessory power relay, behind the “A” panel , where pyrometer and cigarette box is.

 

I like your reply but will share this. If it was easy would they not have already fixed this. Worked with companies that payed me to inspect used trucks to purchase. One bought a group of used Volvo's. One of those Volvo White GMC tagged trucks had a temp gauge problem. Blew that off as everything else about this truck was awesome. That truck turned into being an electrical nightmare over the years.
Maybe he can buy that and come out with a super deal. Maybe swap that relay and see that fixed the problem before you by it.

 

Electrical gremlins can either be stupid simple or a living hell, two seconds with a test light and a jumper wire would tell the whole story of this particular case.
Water never worries me in electrical and the wiring in these old trucks is good. The remnant signs of a door swinger with a pair of dykes and too much time would merit running away.

 

I've owned many of these trucks and it is always the relay behind the left panel behind your steering wheel and it's easy to fix

 

And if you're having trouble figuring out where the water is coming from or you like preventative insurance take that new shiny relay, permatex the holes in the top of it and CAREFULLY drill a vent hole in the bottom... This will help keep the water out and let it drain if it happens to get wet again. Those are not cheap new and they'll last maybe a couple days if you're still fighting water.