Inspect the plug and wiring, it's not quite enough for the application and overheats and melts. (I just had this happen on my left one.)

MB sells a repair kit for it, list is like $5.50, though my closest dealer quoted $11.70 (typical...).

The connector for the bulb in these units is TWO wire, and that's what the replacement is, too. Apparently the SL driver has a twin-filament bulb in his assembly.

Good luck.