Do you need to discharge the AC unit when replacing the heater core on a 1991 Chevy S-10 Blazer?

No, you shouldn't have to. The only thing you should do under the hood is remove bolts from the heater core from the firewall and unhook the hoses attached to the core itself. Your heater core may be located behind your glove compartment. == Answer== I went ahead and tried to replace it myself. Sure enough, no AC unit in site. The heater core is located under the dash on the far right. If you remove the back panel in the glove box as well as the stereo, you do not need to remove the dash either. Taking the right side bolts out helps give you a little more room, but not necassary.