I know what the problem is at least on mine. There are brackets for the tonneau cover that mount permanently to the corners on the frame just below the window. Before I installed those, the windshield wiper had clearance. I installed them when I first used my Tonneau cover. The brackets cause the bottom of the rear window to push out slightly, and not allowing enough room for the wiper to clear. I tried a bumper stop on the rear light, touching the rear window frame, but it didn't work. I think if I put rubber washers between the rear tire/third light assembly, it might move it about 1/4 inch allowing wiper clearance.