Air thru dash vents intermittent on 1999 Mercedes-Benz SLK230

When air flows thru the dash vents, my AC works great. The problem is that I can start the car and the air flows, if I park for a while and start the car again sometimes I get air and sometimes not. Sometimes I start the car and no air flow. It may come on after I have driven for 10-20 minutes. What controls the fan and what could be the problem?

Asked by for the 1999 Mercedes-Benz SLK230