When you post with questions, we need to know what year truck you have, and what engine. Is it 2WD or 4WD? I see on your profile page that it is a 94 and I see its 4 WD. A 1500 I assume. So I know that my 97 has a different AC and heat control system than yours does, but whe I put mine in defrost, my AC compressor kicks off and on, no matter how hot I set the temp, and how cold it is outside. You could try pulling the wire connector off of the AC compressor, and see if that helps, although if there is steam, that is a leaking heater core, most of the time. Hes there been any stop leak added to the system ? What shape is the coolant in ? is it nice and green, or red, or is it rusted ? If everything is nice and clean in there, you can try some alum a seal stop leak. If not then you need to do a flush and fill with new antifreez and fresh water, then do the alum a seal.