My indy said it wasn't the trunk cabling, but I'll check it again to be sure. Whereabouts does the wiring cross into the trunk, it's not immediately obvious from just looking at it.

The lights aren't flashing like the seatbelt warning lights, they're flickering as if there's a loose connection or short somewhere.

I did trace the lamp control device, as part A1265420132 on the StarTek Info online EPC, but it's not the cheapest of parts on FastLane.

No MB module is inexpensive, but ...
 

if all of the affected lamps are controlled by it, then it's a candidate for the problem.

On mine it was the driver's side (left). The wires come out of the trunk rails here. You might have to remove the left side trunk liner to get a good look at it.