1990 Surburban rear window tailgate problem

I have a bought a 1990 surburban off of ebay, and I just got it and i am having problems with the rear window. Do you you have to have it shut to make it work, are there contacts. It has power to switch and motor works part time. Acts likea ground problem. Anyone know if tailgate has to be grounded...Or any other info. Also is it suppossed to go up or down with the key in the back of it.

I have a 1990. You can open and close the window with the key with the tailgate up only. You can use the switch inside but mine does not work. I have to use the key. The motors are known to fail and thus have to be replaced. With or without the key, the window will not move with the tailgate down.

Michelle