I am absolutely astonished at some of the crackpot "solutions" for this bulb eating problem. I just happen to own an '01 C320 which has never burned ANY bulb out after 3.5 years. BTW, I assume you don't live in the US since your car would still be in warranty if you did.

The suggestion about overvoltage doing it can be dismissed because there would be far more problems than just a dead lamp. In any case, 13.6V max is normal when charging in all cars. I this were my car, and knowing that the left side lamps are failing. I would find the ground termination for that lamp assembly. That's the brown wire in the plug on the bottom of the lamp which disappears into the wiring harness descending downward behind the trunk liner. I just looked at my own car to verify this. It's likely that the ground wire is terminated on a ground stud somewhere nearby (a current flow diagram would tell you where, if you had one) and it may be loose or corroded (hard to believe).

This will produce a varying current flow which will kill the lamps in no time. Halogen headlight bulbs are particularly sensitive to high or varying resistance grounding, by the way. Other end of the car, of course.

You can remove the trunk liner panel behind the taillight to eyeball the plug but the entire side panel may have to be loosened to follow the harness very far.

I'm a bit surprised that the dealer tech didn't think about grounding. Or am I?

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Roger E.