It appears that what has happened is the mount which holds the window inside the tailgate has brloken on one side.
Most power windows (I have not seen this particular window, but have many others), will have a crawler gear that running to the motor. The window sits down in a case most times there is a mount on each end of the window. If one of these mounts has either slipped or broken off, then it will allow the window to slide back down as you raise it. You won't notice any issue when lowering it.
This is something that any good window glass dealer should be able to fix. It shouldn't have anything to do with the motor, but the acutal mount.
You can also attempt this yourself, if you don't mind taking the inside cover off of your rear talgate.
Paul C.
