I had the EXACT same problem, but i didnt go buy a new motor. Me and my dad opened up our drivers side master switch and test the current going through all the switch mechanisms. We discovered that for some reason the front passenger windows switch had broken and was constantly sending a signal to go up. Up overides down; therefore, we cant roll our window down. :) Soo we ordered off ebay for like $6 a new master switch, all is well now. hope that helps, nonetheless id say check the currents in the two switches and find out which one is messing up.
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