Hi, I would always recommend a check on a diagnostic tool such as Mercedes STAR to see if any fault codes are stored in the first instance but on this model the fault is almost certainly down to the heater shutoff valve failing - this also usually fails without storing any codes.
When the valve fails it usually fails in an open position allowing hot water to constantly circulate around the heater core
This results in hot air always coming though the heater even when set to cold temperatures
If there are no fault codes stored then I would have the heater shutoff valve replaced as the next step
This is located under the wiper mechanism which needs to be removed - it is tucked under the brake operating unit on the drivers side
It is a bit tricky to access and remove when replacing for the first time!
Any competent mechanic should be able to do the work - it does not specifically have to be MB althouth the part is probably a dealer supply part only
I could probably find a parts diagram if that is any help for you - the actual workshop information I cannot put on here for copyright reasons Im afraid
Click here: Parts Diagram - it is item #30