Thanks for the info. You're right in that, assuming a single point failure, the issue seems to be on the output side of the circuit. However, that doesn't necessarily mean that it's a bad motor.
Personally, I like to know for certain what is broken before i throw parts at it. For example, anytime you have a circuit going from the instrument panel to the door the wires in that rubber boot that connects the door to the cab is suspect. If you have access to a test light and a digital voltage and ohmeter (DVOM) I can walk you through the process of identifying the problem. Or you can just throw a motor on there and see what happens. I'd say you have a 70% chance that the motor fixes it.