Passanger power window won't go down, but will go up.

The clue is "voltage drops off". Think of standing on a garden hose and squashing it 99 percent closed. With the nozzle turned off, you'll still have full pressure at the end of the hose. That's like the full voltage you're measuring. Open the nozzle and water can't get through fast enough so it just dribbles on your shoe. A pitted switch contact will do the same thing. Digital voltmeters draw almost no current so that restriction to current flow has no effect on the reading and you get the full 12 volts. Try and draw some current through that restriction by activating the window motor and that much current can't get through so the voltage, (electrical pressure) drops off.

For a passenger window current goes through both the passenger switch and the released part of the driver's switch so either one can have a bad contact. Also look for frayed or broken wires between the door hinges although the symptoms will usually be a little different at each switch. To add to the misery, GM used to build in relays into some of their switches. If you still have the more simple, reliable heavy switch contacts, you can usually clean them with some fine sandpaper.

A lot of newer vehicles need insane computers added to the circuit to make them work. In that case there may be relays under the dash that can be swapped as a test. Sometimes they're built into a sealed box to insure you have to buy an expensive replacement part. The clue there is there will only be three wires on any window switch, the feed and one for up and one for down. The older, more common sense circuits will have six wires for each switch. Each switch has two switches built in and each of those have two positions; pressed and released. Between the passenger switch and the driver's switch, there are four sets of contacts. All four should read continuity to ground when all of them are released. Depending on which switch is pressed, and whether it's up or down, one of those four contacts will have 12 volts applied and the other three will stay at ground.

What you need to do is have someone hold one of the switches in the dead direction, THEN do your voltage measurements. Use the body as ground for the meter, find the wire at the motor that's missing the 12 volts and follow it back through any switches to find where that 12 volts isn't making it through a switch contact or wire.

That's not actually a fix, and you sure wouldn't pay a mechanic to wiggle parts. That is, however, a legitimate problem and method of finding it, but you really want to know exactly which relay is responsible, then the proper fix is to clean the terminals on the relay, and squeeze the terminals in the socket so they make better contact. With anything electrical, you always want to know exactly where the problem is and what caused it so you can prevent it from happening again or to adjacent wires. Wiggling a plug causes the terminals to scratch clean areas of contact, but whatever caused the corrosion and / or looseness is going to continue happening.

We had a GMC truck donated to our school after no one could fix it. I used that a lot for electrical training, and one of the biggest problems I had was preventing students from poking voltmeter probes into those terminals. They are not very strong and they spread easily. Every year we had intermittent no-crank problems, and every year the kids learned how to replace those terminals.

I had "bugs" built into the window circuit too, but our truck didn't have a relay or any computer controls for that system. When a single relay is used, it is to handle the high current so the ignition switch doesn't have to do that, and develop overheated terminals, which is pretty common. The clue to a bad contact on the relay would be the entire system is dead or intermittent. The goal would be to get it in the dead state and keep it there so we can take the voltage readings. In this case we'd find 0 volts at all the switches. Thank you for adding your findings. The relay and a few wires are all that would be common to all the switches.