You have somethings wrong with your system. After all, you are at 204,000 miles and now 12 years old. These systems typically don't last that long without repairs. Could be any number of things from defective vacuum actuators under the dash and two of them requiring removal of the dash.
Or it could be pushbutton unit, or low refrigerant, or A/C compressor etc. etc.
Sounds like your A/C is not working.

Take it to a independent MB shop for diagnosis.

Enjoy the car, but be aware that these cars must be maintained.

Steve