Air Conditioning Problems

I have a 1989 Chevy S-10 Blazer, I bought it in 2003. The air conditioner has never worked since my purchase, and I don't know why, and with summer approaching again, I'd like to try and get it fixed
this year. Here is my problem(s): when I put the selector switch on A/C or on Defrost, the fuse blows, either immediately or very soon thereafter. So, I have to keep it on either Vent or Heater. However, when it is on Vent, I get some air blowing out of the vents, and some blowing out on the floor. When I put the selector on Heater, I get some air coming out of the front window defroster and some blowing out on the floor. The blower works fine on all 4 speeds.
Any help would be really appreciated. I don't know much at all about auto a/c systems.
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Exactly which fuse is blowing?

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The heater a/c fuse.
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