Rear wiper quit, now defroster not working either on 2005 GMC Envoy

My rear wiper quit working a couple of weeks ago and now the defroster isn't working either. Thought it has something to do with rear hatch replacement after an accident back in May, so went to the body shop where the repairs were done. They replaced wiper motor and checked fuses and "boxes in back" (whatever that means) and they couldn't find the problem. They said tester on switch kept flipping back and forth between polarities--I don't really know what that means. Any ideas?

Asked by for the 2005 GMC Envoy
I would suspect a damaged "ground" wire in the area of the rear hatch. This type of problem can develop where the wires pass from the rear hatch to the body. The "ground" wire for the rear hatch is attached to the body behind the trim panel on the left side of the hatch opening.
1 more answer , 1 more comment
Answe #1 is correct. Go back to the repair center you had work done. That would be my first stop
Well, they say they checked the ground wire and all other connections in the rear hatch (as well as replaced the wiper motor.) I've asked my husband to double check the ground wire also, but until he gets around to it--if that checks out do you have any other ideas? We did have a remote started installed about a year ago. The body repair shop is suggesting that this problem may be related to something they did with the wiring in the remote starter system. Could that be true? I don't understand how it could affect only the rear wiper and defroster. I certainly don't have the $$ to take it somewhere and have them check out every electrical circuit! Thanks for any ideas.
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