The PSE module is in direct control of the rear light. It also accepts inputs from the rear doors, but the actual rear domelight switch is part of the roof module. That module communicates its desires over the interior CAN ( CAN B) to the PSE. You might try disconnecting the CAN connections at teh roof module to isolate the switch input, but that connector also includes the power to the module so all other functions will stop.

The best way would be to monitor all of this with the diagnostic computer.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician