A while back I noticed that I couldn't open or close the rear passenger window from the main switch in the driver's door. That window works perfectly if I use the switch in the rear passenger door, so I know the motor works fine. I swapped in known good window switches in both spots (driver's door and rear passenger door), and it didn't help. Looking at the wiring diagram, I can't figure out what's going on. If I have an open anywhere between the main switch and the motor of the problematic window, the switch on that door shouldn't work either, since it would need to ground through the main switch.

Electrical stuff isn't my strong point, so maybe I'm missing something obvious, any ideas?

For reference, the diagram that I'm using is on page 13 of http://www.volvowiringdiagrams.com/volvo/240%20Wiring%20Diagrams/Volvo%20240%201991.pdf