I was having this same trouble on my '94 2500 diesel truck. About drove me mad, wouldn't "Stay Broke" long enough to fix it. One day it died at an intersection. I had some folks push me over to the side & I went to the parts house & bought a test light, a small adjustable wrench & a small pair of vise grips. By Damn I was gonna find out what was wrong with that truck before I moved it.
Long story short that truck fed voltage to the cab by an external wire at the Pos term on the battery. At sometime the cable had been changed by PO & the connection to the little pigtail on the cable had been made not with a butt connector but with a female Bullet connector!!! It was right there in front of me all the time! They had crimped that bullet connector between those wires just like a butt connector but of course there was no way it was gonna stay tight!.
Live & learn.
