Well, I guess it's official....I'm writing to myself on this thread.
Today I installed a new door jamb switch that I got from O'Reilly's, after diagnosing the problem last night. The part was made by BWD and I do NOT recommend this brand. I ended up having to add a washer to the switch because the threads on the new unit didn't go close enough to the head.
After getting some gas tank straps at the pick-your-part yesterday, I wanted to put the 16 gallon tank up under the truck so that I don't have to keep moving it from the passenger seat to the bed, bed to passenger seat. I quickly found that the mounts don't line up against the long bed frame....although I did luck out and found one bolt hole in the frame to mount the front strap to. Keep in mind here that I am only STORING the tank there. All of the holes in the tank are plugged, there is no filler hooked up to the tank and no sending unit installed or hoses.
I then moved on to my attempt to get the coolant gauge to work. I ordered an AC Delco gauge sending unit from Amazon and put that in the block. I was happy to see the gauge move once the engine was started....but then normal operating temperature was right at the "H"....I'm thinking either the gauge is off or the needle needs to be adjusted (if possible). Who knows how many times that thing has been messed with.
After that, I moved on to the dome light not working. After replacing the door jamb switch, I saw there was a light under the dash that I didn't know about. That was a nice surprise. I quickly found that the orange and white wires supplying power and ground to the dome light, that run along the driver door sill, were corroded through. So I picked up a section of that from a donor truck at pick-your-part, soldered it in, wrapped it in heat shrink tubing, and got a new L90 bulb. Works like a charm.