I have a 1999 M Roadster. Recently, I noticed that when my car is at idle, the blower will not put out hot air. I noticed this when I was stopped for a railroad crossing. It was putting out hot air before I stopped, then over about a minute it cooled off to outside air temperature. A few seconds after I started moving again, it blew hot air.
This does not happen with the air conditioning. Any ideas what might be acting up?