It's probably a relay unit that controls the central locking. Some cars will have separate units for the doors, boot and fuel cap and some are combined. The relay unit (switch) controls electricity to the locking mechanisms (small motors) which rotate a locking pin. It very unlikely that the boot and fuel cap locking motors failed at the same time, so check the relay units (switches). Each relay will have it's own spec. so don't swap them around. You'll probably have to end up buying the relay at a Mercedes original parts dealer.
Robert, June 2007
