Is your truck stock or have you or someone you know been messing with it, either installing a stereo or other electrical items ?
That would be my first guess.
If it is stock, then you are just going to have to spend the time, and find the cause of the short. There is a wire casing that has rubbed against something hot, or sharp and the covering is bare enough for the wire to be exposed and it is shorting.
You are going to have to trace wires from the switch to the lights.
That model was an electrical nigtmare! If u were Near hou. Tx. Ild be able to run a diagnostics on it so you know exactly what was causin th blo. I would start cheap and go up.fuse/get one 10a bigger if it was ten amp go 15amp first.then work ur way up.Relay,and so on.good luck!555