i have an 89 gmc r3500 that the widow motor works prefectly most of the time but every once in a while the window will not go up or down but if you wait a little while it will start working agian.it has 4 doors and only the passenger door does it all 3 other are perfectly working all the time even when the other is not.i was wondering what should i check first before replacing the more so i dont waste money

Spitballing here:

If it is a wiring issue, you should be able to have the door apart, wait for the problem, put a test light on it and see if power is getting there when the switch is activated. If your truck is in good shape (not all rusted and beaten), I'd say chance of this is moderate.

My first guess though: When this problem is happening, push the switch and tap on the back of the motor with a hammer. If it starts moving with one of your taps then the commutator in the motor has some dirty spots on it. Go get a new motor, swap it, and problem should be gone.

When it doesn't work, it doesn't work from the master switch AND the passenger door switch?

If that is the case, then I would pop out the door switch and have a test light handy......back probe the switch to the wires going to the motor with the test light and command window up/down from either switch.....if it lights up but window does not move, need a motor.....

If it doesn't light up, then more testing is needed...