Liscense plate lamp is a good possibility, and so is a lamp you replaced (or the PO replaced) and didn't replace the other lamp of the pair at the same time. I don't know how sensitive the MB is to exact lamp current, but the Volvo will give fits if the lamps are exactly the same, and even then some brands will give an intermittant lamp out signal.

Check the condition of the lamps that flash too fast -- could be some corrosion at the bulb socket, or mis-matched bulbs (one brand new, the other nearly worn out).

The senor compares the current draw for the two lamps and will turn the light on if they differ very much. Volvo uses a set of magnetic switches, I don't know how MB does it.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!