There must be some sort of system to say the windows are open and it's causing interference. Maybe electric window control box or part of the security system. Deffinitly a wierd head scratching one though. It might be worth checking the speaker connection on the passenger side though. You could try giving it a spray with electrical contact cleaner then puting the plug on and off a few times and see if that helps. If I think of anything else I'll let you know.