The reason the left side don't work is often the circuit as it passes through the seat belt module is open. The passenger side light switch goes straight to the light. Pulling the module has no effect on the right side but makes sure the left side don't work.

Look at the wiring diagram and follow the door switch circuit for the left side. The reason this all happens is they use the same switch for two different circuits: dome light and seat belt warning.

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