You should also consider the possibility of water in the trunk area, which will mess up the module back there. It has happened often.
If the car has sat out in the rain a lot, you better tear the trim out of the back and start looking for water in the wells under the mat and in the wheel wells, back by the bumper.

If there is water in the harness or modules, this could be a long path back to health for the car. I hope it's something else.