Heater broken

[adrenelin500]

I have a 1982 300sd that i'm in the middle of fixing up. We'll to add to my list of things to do now I have a broken heater.For whatever reason last week the heater stopped blowing air(cold ,warm or hot) nothing ,nadda.The only thing the heater does is make a faint hissing sound when turned to full cold.Can someone please tell me where to begin.I have two manuals, one has the section torn out and the other dosen't really seem to talk much about my car whatsoever so there no help. Also I had my door locks die about a month ago (the pump tested fine). Could there be a link?Thanks in advance for any help.