Ballast Resistor is a piece of porcelain mounted on the inside of the driver's fender. It has two wires running to it, which can easily be jumped together.
Power to the pump is run through the resistor after start up. If the resistor is bad(open), then as soon as the startup process is done, it will switch to it, and the pump will quit.
If your truck is an early 87, it's possible it won't have one, and therefore that won't be the problem.