I have a 1998 S320 and just noticed the the heater is not working. Living in CA didnt require me to use it until just a week ago.

I checked and both drivers side and passagers sides, both blows cold, I raised both tempatures individually on the climate control panel. No help. Hit defrost button and it goes into full blowing mode but still blows cold.

Any suggestions where to start?

Thanks in advance.