You hit the magic clue; both motor wires have 12 volts at the same time when a switch is pressed. You are missing the ground, and the common cause is a broken wire between the hinges. If I'm right, you will find the passenger side window also doesn't work because its two wires go through the switch in the driver's door, then to ground through the same ground wire. The following is a copy and paste from a previous post:
Suspect broken wires between the two left door hinges. The following repair pertains to Caravans, but the procedure applies.
Peel the tape open on the wire harness between the two left door hinges. Remove the door trim panel, unplug all the stuff like power mirrors, windows, locks, speakers, and courtesy light so you can pull the harness part way out of the door. Pull as much harness as possible out of the "A" pillar.
There can be up to 22 wires and some of them will be the same color with the same color stripes. If you mix these up, everything will still work, but you might pop an automatic resetting circuit breaker if the right power window is operated in different directions by two different people at the same time. To avoid confusion, start with the broken wires, then move on to the wires with cracked insulation.
In 1990s Caravans, the replacement section is 11" long. Slide the rubber grommets back to expose the sections or wire that were inside the door and "A" pillar. Cut one wire in these two sections. The replacement wire must be the same gauge, stranded, and 22" long. The color is not important. Strip about 1/2" of insulation from both ends of the replacement wire, and both ends of the original wire. To splice the new wire in, slide the strands together and push down any strands that stick up. Solder the connection by heating it with a soldering iron on one side and applying the solder to the other side, never directly to the iron tip. The molten solder will flow toward the heat source. When it cools, check for any sharp points. If you can feel them, flatten them with a needle nose pliers so they don't poke through the heat-shrink tubing. Slide a 1" piece of heat-shrink tubing over the splice and warm it with a match or lighter. Don't over-do it as the tubing will melt and split open. Also, don't use electrical tape. It will come off in hot weather, and is a sign of laziness.
Slide another piece of tubing onto the wire, then splice the second end. Install the heat-shrink tubing, and the first wire is done. When all the wires are done, wrap the harness with tape. Electrical tape will crack apart in a short time. Friction tape works well, but don't make the bundle of wires real tight. The wires will want to slide over each other a little when the door is opened and closed.
Slide the grommets over the ends of the tape to help hold it in place. Slide just enough of the harness back into the door to reconnect all the plugs. All the splices on this end should be inside the door so they don't flex when the door is opened. By splicing in 22" long pieces of wire, the new section of the harness is twice as long as it should be. Slide the rubber grommet for the "A" pillar down far enough that only 11" will be between the door and "A" pillar. Push the other wire into the "A" pillar and coil it where the parking brake pedal won't interfere with it. The reason for the extra wire is to get ready for the next time this repair becomes necessary. Just pull the harness out and you will be half done! No need to make the splices on that end.
Thursday, March 26th, 2009 AT 1:31 AM