I'm new here and since I'm 13, you really shouldn't take any advice from me (since I'm not a professional) I am knowledgeable about electronics and even more knowledgeable about HVAC (heating and cooling) equipment.
I, however don't know much about automobile HVAC systems, although it should be the same principal.
My advice: Look for the relay (like Warlock III said) and that may be your problem! The relay is usually a little black box with four wires hooked to it (the wires may depend from it being a SPST, SPDT, DPST, or DPDT type relay.)
I can probably help you find the relay if I had an electrical schematic. I'll try Google and other things since it's the weekend.
I'm glad to help in any way, shape, or form. Like I said, I don't know much about car's nor am a professional. I just happen to know a lot about HVAC equipment.