Try to close the door with the switch on. That may release the motor/switch giving you a TEMPORARY fix. You can also try to clean the switch with contact cleaner.
Most likely you have to replace the whole assembly. I did the same in my car. A DIY job, once you figure out how to remove the door skin.
The other possibility is, one of the wires that provide power to the rear window is/are broken, at the door jam. Check if you get power to the switch.
Good luck!
J.H.
86 300E