1987 corvett both windows went up by themselves now they won't go down I have 12 volts on both wires to the motor is there a relay some where

Check battery must fully charged, check starter fuse and relay Check 12 volt wire from ignition switch to starter relay in the fuse box ,then check 12 volt wire from starter relay to starter motor solenoid if if it's everything is ok then change the starter.

Check connection battery,could be starter.

If this is any help you need to check the voltage with your meter between the two wire, do not put one lead of your meter to the plug and then the other to a ground, that's not how it works. Again you need to put one lead of you meter to on side of the plug and the other lead in the other, now try you switch up and down you should get at least a 12 volt reading on up and down, they usually have a really that switch the 2 wire so depending on weather you are trying to go up or down determines witch wire has the 12 volts and witch wire is the ground at that time, so if you read between the two wire you should get 12 volts if you don't then ether you switch is bad or there is a bad relay. Or you may have a bad ground, it look like you checked and found that there is 12 volts at the plug you have lost the ground side with in that Circuit So That this is so long I am a talker I hope this helps.

check fuse first/ if good check wires at the switch for voltage if all good check for voltage going to window motor if no voltage replace switch if you have voltage (12) v change motor

i had the same problem, its the fuel pump relay, pull the relay, pop off the plastic top, insert a shim so that the points are always touching, the other thing it could be is the pigtail connector coming off of the fuel pump inside the tank, chronic chevy problem

You need to check using a multimeter on the solenoid for the operating voltage when the relay clicks on activating. If there is voltage then the solenoid is faulty and if not the relay contact points have burnt out and so does not give the connection for switching the voltage.In either case get yourself a multimeter or a 12 volt park lamp bulb to check the voltage on the solenoid.

does it click or grind when pushing window button if so the window lift track may need replaced, if there are no sounds then disconnect the wires from the lift motor and with the ignition on so power is sent to switch, push button and check wires to 12 volts dc. If no power I would replace the driver side switch after checking the wires for defects. If there is power then I would re-install wires to motor after turning off key and try button again with power on. replace motor if it doesn't work.

Multiple issues here??
Why did the motor quit??
Did you check the engine control fuse box for issues? It is next to the battery?
The is an imprinted 'map' under the lid for direction. Check all fuses and relays and post back.

Sounds like you are missing a good ground.

Here's something to try. There should be a single black wire on the connector for the master window switch. this is the ground for all the windows. check for power by back probing that wire with the connector plugged into the switch and a window switch in either the up or down position. If you see 12 volts than the ground is bad. Use a jumper wire from that wire to a known good ground. If the windows start working than you will need to trace that wire to find the break. The most common would be in the harness in between the door and the body of the car.

If you don't see 12 volts with a switch in the up or down than remove both connectors from the switch. test continuity between the pin that the black wire goes to an any pin that the 8 wire connector plugs into. There should be continuity. If there isn't that the master switch is bad.

If you have any question let me know.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Either switch, motor or even the Window Control Relay. Before performing any tasks ensure that the GND (Neutral) exists everywher it's supposed to (motor & relay especially)
To check the motor itself, disconnect it from any wiring an measure if there's any resistance betveen motor connection pins. or when its disconnected force some 12 volts to the motor and it will move up or down if it's fine. If it moves then go to the switch, and using multimeter measure if there comes 12volts. if it does check if it goes out from the switch when pressed. If it does, and you've already tested the motor and it was ok so the problematic is Relay (might be open circuit, defective soldering on the relay's circuit board). f nothing comes out from the switch just replace it.

If you need any further assistance just let me know.

Best of luck