broken wire between the switch and relay? or relay and motor?
I have to take it the vehicle your talking about is the '85 K10/350.
The wires commonly break in the section between the door and the body. That's the point where they flex every time you open and close the door. As the wires get old they become brittle and break. You can unplug the wires from the fuse block and do a conductivity test on each wire to see if you have a broken wire. If you find you do have a broken wire you can feed the harness back through the door, through the boot and start your repairs.
Check to see if the wires are brittle. If so, which they almost undoubtedly will be in an '85, it is a good idea. in my opinion, to splice in new wires in the section that flexes, solder correctly and shrink wrap to avoid having to do it over and over again as each wire breaks. Be sure to use the correct gauge of wire. At least splicing in new wire is my recommendation if yours are brittle., YMMV.
While this may not be the cause of your problem it's the most common problem I've found causing the issues you've described.
Easy to check and very common issue.
Good luck.