yep i'm guessing that the heater core was busted and the po didn't bother putting in a new one. you have to take the heater box out from under the dash to put a new one in, there are studs coming through the firewall on the engine side i think theres 4 of em, one behind the valve cover, to the left side of the battery tray and two under the battery tray, remove the nuts from them and that is all that is holding it in. you might need to remove the battery tray to get to the ones under it but that should be easy. you will also have to take the cables loose from the heater box. then you pull it out from under the dash. after that you take the back off of the heater box and drop your new heater core in. when you put it back together make sure all of your air cont. flaps are in their correct positions. after that if you want some real heat go and get you a chevrolet k5 blazer blower motor and put it in before you re-install the box, it'll run you about $20 for the motor.
