Actually, I think that pump is always running, not only in REST mode. I think it also help to keep the hot water flow constant, so that it doesn't vary with engine revs. Otherwise you have very little water-flow at idle. Or at least I know that on my 190E it was always running, haven't checked to see if it is the same on the SLK.Troll posts deleted.
All that pump does is pump water through the cabin heater to keep you warm without the engine running.