Instrument Panel Gremlins At Work

[Bella79]

I recently purchased a 1987 4 wheel drive GMC shorty automatic with a 305. It has some electrical issues that I have been working on for a month and they are getting the best of me. In addition, I purchased a new to this truck instrument panel that is in excellent condition but to no avail. The fuel gauge pegs at its beyond highest level. The oil pressure gauge does the same. When the ignition is off I can tap the face plate and they go down but not to "zero". The continuity tester is showing the possibility of a short but it is hard to say as the electrical schematic seems to make sense that some of the wires should have continuity with the one I am testing. The water temperature had a similar issue but I replaced the sensor and that did it. I replaced the oil pressure sensor but that didn't help. I can't see any "burnt" wires or evidence of short circuit. I have ordered a new ignition switch to see if that helps.

Also, the dome light and floor lights will not come on with the door open even though I replaced the door switches. They do come on with the headlight switch which I also replaced. The warning buzzer for the headlights left on or key in ignition works fine.

I keep plugging away at it and the wiring is pretty simple but I just can't seem to find the issue. I'm guessing once I do find it both issues will be taken care of. Today the fuel gauge would go to empty when I put on the breaks and was on a downhill grade then as soon as I took foot off brake it went back to being at highest level. Didn't see if oil pressure gauge did same.

Any help is greatly appreciated. I have been reading posts here to see if something would help but it hasn't.

[hatzie]

Nailing the oil pressure and fuel level gauges is an open sender wire. 
If you ground the sender wire it'll go to the bottom of the gauge.

Nailed fuel gauge. 
I assume she's running so the ground is good...  the TBI fuel pump shares the sender ground.
Check the sender wire isn't damaged or unplugged. 
Check the sender wire back to the bulkhead plug is in one piece. 
Check the cluster plug connection and the plastic printed circuit is making contact. 
Check that the spring loaded clips are making contact with the printed circuit and the gauge stud in the cluster bucket.

Oil pressure gauge sender will be down the Left side of the block above the oil filter.   Likely the single TAN wire is damaged or unplugged from the sender tang.

See this post. http://forum.73-87chevytrucks.com/smforugood m/index.php?topic=34110.msg290846#msg290846  Both the oil pressure switch and the oil pressure sender is referenced in that post.