Stumped.. Electric problem or bad motor?

Hello

I have a 2002 Buick Rendezvous.

Just recently the rear wiper has started acting up. When I try to use it, it either works, or it buzzes and I have to manually help it go back and forth to operate. When it buzzes I noticed that the lights on either side of the wiper on the lift gate turn on faintly. Wiper stops buzzing, lights go out.

I replaced the wiper motor with one from the wreckers, and problem remained the same. Unplug wiper motor, and lights stay off.

Thinking maybe a bad ground?
All wires that I can see and get at look like new..
Could it be as simple as just running a new ground from frame to the door?

Got me stumped.

Any help would be great.

Thanks
Paul