I dont know if you have tried, but try the easy dummy stuff first. you know, like checking the fuses in the cab and under the hood. you wouldent believe the hours I have spent trying to find a problem just to find the darn fuse was blown!
If thats not it, bad grounds are usually 90% of the time at fault, especially on old vehicles where corrosion can build up. Check all your ground straps from small ones grounded on the fenders or the firewall all the way to your main ground off the battery. look for frayed ends, broken or crimped wires along the length of the wire and of course corrosion on the ends (take some sand paper or a wire brush to clean it up, you want nice metal on metal contact, no paint).
Now if all else fails, it comes to the pain staking part of tracing your power and ground wires back to the pump with an Ohm meter. Put one lead where you know you have power, and use the other lead to probe the wire testing for Ohm readings down till you reach the pump. Eventually you will stop getting a reading off your meter and thats where your break is in the wire.
Hopefully this helps! this method hasent steered me wrong in the past yet!
