I have an 88 Burb. There are two wires going to the motor. Both are power leads, one for the up direction, and one for down. The motor has to be grounded to work. Put a voltmeter on one lead and have someone hit the button one direction then the other. You should get voltage at least once. Try the other wire, same thing. If you arent getting any voltage from one wire, that your bad lead and could be either the tailgate key switch or the dash switch or a bad wire.
Test the switches like you did the wires. Ground one lead of the voltmeter and put the other on one terminal of the switch (not the power source which will have voltage on all the time). Flip the switch like before and check for voltage like before. If no voltage on one side (most likely the down position since your window wont go down) then the switch is bad and you make a trip to your local auto parts store. If you have good voltage on both sides, then your wiring is bad going to the tailgate. Visit your local auto parts store, get a spool of at least 20 ft of 14g or bigger wire and have fun running it to the motor from the switch.
If need be, I can take some pics and post them, but it will take a few days.