Electrical diagnosis is my specialty. Although a bit novice to "older" diesels like the 6.2L, I am an ASE Master Automobile Technician and ASE Master Truck Technician, with L1 and L2 Advanced Level certifications,as I teach High school Auto Tech now (3 years). Grounds grounds grounds grounds grounds GROUNDS!!! Most of the drivability issues, and electrical/electronic issues I see and fix are grounds! The plus side of that is that grounds are east to fix, even if you don't have a factory manual. If you have a digital volt meter, an easy way to check for poor ground performance is simple. Hook the positive lead of the meter to the negative of the component (for example, lets say it is a blower fan we are checking). To do this, use a paperclip to backprobe the electrical connector (the component needs to remain electrically connected). Clip your positive meter lead to this backprobe, and then clip your meter's negative lead to a known good ground. Now operate the component. If the meter reads more than about 0.5V when operating the device, the ground is bad. Also, if it is a sensor or sending unit, that number is even lower, as many engineers want the voltage drop on a sensor circuit ground at less than 0.1V, and some even specify 0.05V. How to fix that? You could dismantle half of the truck, or, you can just splice into the ground wire and run a new ground. Don't skimp on the wire gauge, make it nice and thick.