My 85 headlight motors have been rebuilt and with new relays and they were working fine and then the right motor started not going down. So I removed it and plugged it in on the other side and it worked fine no trouble at all. Any ideas how to find out what is causing this problem and where to start. I also replaced the headlight switch awhile back
First, try swapping out the motor relays. Even though new, it's possible one of the relays failed. The relays have two circuits, one for up and one for down. Also, be sure that the barrel connector was surely engaged. Sometimes they don't latch together and the ground is lost.
I would suspect that one of the brushes popped off the limiter switch. If you bump the knob with your fist or get it started will it go the rest of the way down on it's own? If the answer is no, remove the assembly to the point that you can remove the plate covering the limiter switch. Without removing it, carefully examine to see if both brushes are still attached to their spring arms. Also, peek up underneath the knob to be sure the rubber D weather seal isn't visible. It should be down inside the top of the case halves.
I don't believe it would be the isolation relay. I don't have a schematic for it, but I believe the up and down switching is done at the motor relay.