05C-Class

I bought an 05 C240 a little over 4 months ago and not until yesterday was it nescessary for me to use the heat. Well it turns out the Driver's side heat does not work. I cracked it above 80 and it blew tremendous amounts of cold air, now the passenger side not a problem nice warm blowing around. Any tips, suggestions, brainstorming, places to begin...As I am creeping up on a cold winter fast. Thanks for the help.