[tincan1966]

If you have a C10 with a 'tandem lock" light on- someones got a bulb in the wrong socket. That is more likely a generator light that should come on with just the key "on" then go out after the engine starts. if it stays on, you may have a charging issue.
Alot of times an erratic fuel guage is attributed to a failing sending unit in the tank- in other words the reostat portion is worn and not always making a good contact , internally. Again, this is sometimes, not always
Another guess would be that someone put a guage cluster in a truck that originally have indicator lights and did not change all the sending units.
the oil pressure is mechanical.
To test the temp wiring and guage- take the wire off at the sending unit. Note if there are 2 wires(lt green and dk green)then it originally had a light- you will want the dk green wire. Ground the wire to the engine block. Momentarily turn the key on-if the needle goes to hot- then the wiring and guage are good. If nothing happens, first check the fuse for either "panel" or "instruments" .

I'd test the speedo as mentioned above- note that you will need to run the drill in reverse, I believe.