Did you ever find your convertible rear window motor assembly?
There is one of these in a salvage yard by me. The motor assemblies are different between LH & RH sides.
If you are mechanically inclined, if the motor itself is open-circuited, you could pull the outer housing off to get at the brushes. Most likely the motor got wet at some time and the springs are no longer capable of moving the brushes against the armature. Taking the brushes out and cleaning them and the tracks they slide on, then put on some good synthetic grease on the slides most often brings the motor back to life.