Ok, lots of staring at the wiring diagram and here is what I've got:

BTW I hate wiring diagrams, I much prefer schematics.

Looks like the ground routes through the drivers side switch, which is missing. Thus, no ground.

Also looks like the motor is isolated from ground, so must operate be reversing polarity (switching both leads).

So, since the passenger side switch only switches one motor lead, bad switch on pass side.