I'm guessing the car will have a canbus network and you'll need the appropriate wiring, and quite possibly a session with a computer to have the car electrics enabled for the towbar wiring.
Mercedes Benz C-Class estate S204 - 04 2.2 Tow bar fitting & electrics........ - carmalade
The main thing to remember on these is you cannot wire the tow bar electrics directly into the existing rear lamp wiring.Serious damage to the rear electric module can be caused by this.Some of the lighting circuits on these are not fused ,they are overload protected electronically within the module.hth