I called Mercedes but all they suggested was sending out a breakdown truck: not really necessary.

I went to the dealer and--shock, horror, probe--they were very helpful. I spoke to a tech called Rob Lewis who said he was a diagnostic tech from Mercedes (as opposed to a regular mechanic); he knew about these cars than I do, which is a first at the dealer (he showed me his own SL55 AMG). Anyway, he has disconnected the headlamp washer pump and booked me in for work. I still think it is either a connection problem at the pump or a problem with the relay (because the problem only happens when the lights are switched on).

I have xenons and our regulations dictate that we have a headlamp cleaning system with such lights.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190