Weird window switch electrical problem.

[Texas 940]

I just purchased a 1995 940 and it has a unique problem. When the key is switched to the accessory position, the windows operate normally. When the key is switched to the on position (as in when the engine is running) the windows are inoperable. It doesn't sound like the switch is malfunctioning to me. The power mirrors continue to work while driving, which makes me think that there is constant power (unless the mirrors are on a completely different circuit). Any ideas where to start the diagnosis?