'99 Focus heater
 

I seem to be having some problems with my heater. Sometimes it works, sometimes it doesnt. I can hear the fan inside go on but nothing actually comes out of the vents or the defroster. If I turn it on full blast, a tiny bit comes out but barely any to make a difference. But then theres times when it works fine like its no big deal. Can someone please tell me what the deal is and if its going to be a pain to fix?