Last year (2012) I had my air conditioning re-.

Not being a Mercedes mechanic this is a generalized answer. First the a/c will hav eto be checked to see if freon is low, then it depends on whether you have automatic air or not-meaning whether you set the dersired temp. It could be eitehr of these things or stuff like actuator s for hot/cold or air direction. So unfortunately your problem is deeper and will need to be checked out. If you just filled it a while a go whenit was regassed as you call it there had to be a leak someplace as freon just dowen't go away without a leak. It shold have been checked for leaks then and repaired. This time if the freon is low again have it leak checked and not with dye, have an electronictester put to it then it will have to be evacuated and recharged and probably the receiver /drier will have tobe replaced. Hope this helps you

Yes it could also be a bad switch. Like I said earlier i'm not a mercedes mechanic and not real familiar but I am very familar wit hh A/C systems and am quite sure what I explained is what your problem is.