I have a 91 420sel,the dome light,memory seat control and seat belt warning light dont work when I open the driver door,but all work with the passenger door.I believe the relay is behind the instrument cluster.My question is,is there a similar relay for the passenger door and location?Is there anyway to test the relay?

Test the driver's door switch first.

The warning module is up under the knee panel by the brake pedal arm.(works both sides)

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

There is only one relay. The general test for a relay is to see if the output is as expected with the proper inputs. From your story, I'm guessing that the output you seek is domelights when the driver's door is openned.

Unfortunately, while the relay is a posible problem, it is not the relays function to operate the dome light. The circuit just passes through the relay.

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