I'm driving a 1991 Chevy C1500 with the 5.7L. I smell antifreeze and it's fogging up the windshield. How do I replace the heater core?
The replacement of this particular half-ton's heater core is a comparatively easy job. Let me rephrase that; it should be an easy job, but you never know.
The book tells me, you have to remove the lower heater case cover plate (right side, under the dash) and the blower motor. Then detach the heater hoses on the engine-side of the firewall (after partially draining coolant from the radiator). The heater core should then come out after the removal of a few more attachment screws.
The labor-time book tells me it pays 2.5 labor hours, not including evacuating and charging the air conditioner. Which means the air conditioner's evaporator may also have to come out, or at least be disconnected to gain access to the heater core. And you can't just detach an air conditioning component and release refrigerant into the atmosphere. There are federal laws dealing with that kind of stuff. You'll have to take it to a shop with refrigerant recycling equipment to discharge, go back to your garage and do the job, and then go back to the shop for a thorough evacuation and recharge of the system with R134a.
Seeing your C-truck is a 1991 model year, and came from the factory filled with R12 refrigerant, you'll also have to install a R134a conversion kit, if it hasn't been done already -- current cost or R12 is ridiculous.
If all this sounds to you like a real big job, sublet the work to a local shop. Of course, you can always bypass the heater core with a section of heater hose if you live in a warm climate. Good luck.