The number one answer is what Carl said, be darn sure that there is a COMPLETE ground path to the battery. Then check for hot at each socket with key on, if hot...check bulbs, if bulb works at a remote test place (battery), it's the ground. If no hot, follow both hot wires thru whatever fuses are in the circuit and check those fuses, maybe they are on the same fuse. No one ever knows what a PO has done, especially wiring. The senders operate on ground also, so motor has to have good ground to Battery and battery to chassis and chassis to frame. All shiny clean ground spots. If you have to, run a wire from bulb socket direct to battery ground terminal, that will cut to the chase in a hurry. My money is on Carl, as usual.
P.S. One of those ice pick testers sure helps running things down.
