The down wire is defective.. . Seriously, how did you determine the switches were OK?? I am not sure, but I think these switches are series/parallel wired which means that both have to be intact for the windows to work. I'll have to check that. However, all the switches do is reverse the polarity of the current going to the motor for up and down function. If the front switch is dead and you've tried a known good switch there, and this failure occured suddenly. (did it??)then start at the console and check voltages for the center conductors for that switch. Should show 12 volts with key on. If no juice, then the problem is further downstream. if you've got the 12v, then it's a crappy switch.

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Jeff L
1987 300e
1989 300e
1987 BMW 325