Electric mirrors

I don't know as regards MB but from experience with SEVELs there are a big variety of mirror variants and quite a complicated electrical connection to the mirror to cover adjustment integrated flasher temperature sensor (in some cases) integrated aerial (in some cases) and motorised retraction if fitted. An OEM mirror for a Citroen Relay can cost upwards of £500 and MB parts are famed for being inexpensive.

As regards the wiring, in build some manufacturers have a lot of wiring built into the loom by default to cover a variety of option choices but others only wire in for the specifics. A good main dealer parts department with the benefit of your VIN number may be able to advise you if it is possible. However sourcing mirrors new may cost a four figure sum so a reputable breaker may be your next option. 

Mercedes don't fit generic wiring looms so it'll be very unlikely there will be spare connectors.

Thank you BlueVanMan and MollysMummy.

BlueVanMan, did you mean expensive not inexpensive?

MollysMummy, Ah well! I'll just stick with what I've got. They don't need adjusting that often anyway.

sorry I meant expensive but you may already know that they are !