Pull the plug, the two poles on the plug reverse themselves depending on which way you push the switch (reverse the current and the motor reverses). A simple trouble light between the two poles will tell you if you are getting power and ground, the light doesn't care which way the current is flowing. All your windows will stop working if the fuse or the circuit breaker fails.
Sometimes the gear gets jammed on the cable that operates the window or slips at the full down position. Pushing up on the window, working the switch up and down and rocking the window a little left and right can unjam things on occasion.
If you are getting current, with both switch positions (up and down), try pushing the switch down and see if the motor is trying to move the window.
I'm thinking the up wire for the motor is fatigued through. It happens often just where the wire leaves the small plastic junction box on the motor.
Your going to have to remove the motor to fix it (quarter inch drill or maybe a 5/16" I used a 6mm). Drill out the rivets that hold the motor in and for the galvinized cross brace. The cover comes off the back of the junction box, then you can pull the spade connector and cut out the piece of fatigued wire and crimp a new spade connector on there. Leave the top off of the junction box and push the spade connector in the back, don't even try and find one of those French backwords spade connectors they use OEM. Bench test it and put the motor back in with short 1/4" bolts and nuts ( a little Loc Tite or lock washers). The motor takes a little talent to remove, kind of like a puzzle. But you relly don't have to bend or force anything. Removing the rubber tube at the bottom, can be helpfull, but it is heck to get off of there.
If worse comes to worse, take the big nut off of the bottom of the window where it connects to the drive cable and make yourself an "S" hook type of setup to hold the window in the up position (I do that anyway when I'm taking the motor out, to keep the window out of the way). I make a hook in some welding rod (coat hanger is about the right thickness), hook it at the bottom of the window and pull the wire up on the inside (between the window and the inner rubber), when the window is up all the way up bend the wire down towards the inside and cut off the extra. I usually end up with my "S" hook somewhere near the front of the window.
If the gear drive is slipping on the cable in the full down position. I'll give you some tips on getting a little more service life out of it. There are some tricks.