There is a known problem with Fauly PSE (vacuum locking pump) which also controls the domelamp in a SLK. Check for waterleaks
in the r side of the trunk which can get this pump wet and cause corrosion and faulty operation.
Remove the spare wheel and jack. Remove the foam partition
the jack sits in. The PSE pump sits in a foam block behind there,
and this foam will soak up water killing the pump.
The water leak usually comes from a faulty seal on the rear of
the vario roof that seals against the trunk lid when closed.
The dome lamps also come on when the trunk lid is open, so check for a faulty trunk lid latch microswitch.
The door pin switches on the front doors should be checked as well.