I’m trying to figure out what triggers the ground wire so the light comes on. The wire between the hatch and the body may be severed. I need to check continuity but I’m not sure that’s it.

The wiring schematic looks like the connection may go through the wiper motor

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1985 300CD, CA
172 Anthrazitgrau/Palomino 154

1985 300TD, CA
040 Schwarz/Palomino 154/3rd row seat

1985 300TD, CA
473 Champagner/Palomino 154/Cargo cover