In the W124, all the exterior lights run through N7, the "Exterior Lamp Failure Monitoring Unit." Each lamp is connected separately to N7, which is located in the relay/fuse box. In my '87 300D Turbo (W124, OM603), N7 is a long black rectangular module with 34 pins in three groups and occupies the left-most row in the relay section, that is, closest to the left front fender. Your '90 model should use the same unit (I think).

N7 plugs into three separate connectors in the relay box: one with 8 pins, one with 10 pins, and one with 16 pins. The right tail/parking lamp is connected to pin #10 of the 10-pin socket with a gray/red wire.

Although it is unlikely that N7 has failed, it is not impossible. By removing N7 from its sockets, you can get at the individual pins in the sockets. Apply +12 volts to the pin described above and the right tail/parking lamp should illuminate. If it does not, there is a wiring or ground fault. Check the wire to the pin in the relay box (but it will be difficult to trace through the cables in the car). The right tail/parking lamp is grounded at W7, in the right rear corner of the trunk, behind the trunk lining panel. If the lamp lights, the problem may be in N7. The only practical way to check is to plug in a spare.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970