Had this a few months back and came to the conclusion it was the rear left cluster fault. I sourced a cheap one from ebay, replaced and all fine, well a least it was this morning..........
The lamps are sealed so if one part fails the whole thing has to be replaced. Not the cheapest thing in the world but there are quite a few used ones floating around after people had the clear lamps fitted.
Apparently, this is a common problem for people who are a little bit overzealous with the jet wash! Something about water managing to get in to the unit from the top ... or so one of the local workshop guys told me ....
