The heater core hidden deep inside your climate control system was not designed to last forever. The first sign of heater core failure is a puddle of sticky water on the carpet accompanied by the stench of warm antifreeze. When the heater core springs a leak, most people will just bypass it by splicing both heater hoses together, but replacing the heater core is not difficult and can be done in an afternoon.
This '66 Chevelle's heater hadn't been used in some time and was in need of a redo., so we ordered replacement heater system components from Classic Industries and National Parts Depot. The complete system was removed from the car for the overhaul, which included cleaning and lubricating the control cables and replacing the heater core.
We first unbolted and removed the passenger-side inner fenderwell in order to access the heater case assembly on the firewall. There are five 7/16-inch-head perimeter nuts, one 3/8-inch-head sheetmetal screw on the bottom left side, and one 7/16-inch-head machine screw up high on the right side of the motor bolting the case assembly in place. The 3/8 screw and 7/16 bolt had to be accessed from inside the passenger-side wheelwell. On early Camaros, this means removing the right front fender as well. All five 7/16 nuts can be unscrewed from the engine compartment. Drain the cooling system and remove both heater hoses if they're still connected at this time.
With the outer case assembly removed, you can pull the heater core box out from under the dash. It may take some interior gymnastics to get it out, and you may have to remove some factory brackets bolted to the firewall that help support it. Be sure to clean and lubricate all moving parts such as the hinges, control rods, and original cables if they're not broken. Hang the cables from one end and slowly drip light oil down the cable, moving it in and out of its sleeve to distribute the oil properly.
After the bench-top clean-up and rebuild, reinstall the case assembly, sealing it to the firewall with black RTV silicone or 3M Weatherstrip Adhesive. You should also flush the engine's cooling system and pour in a fresh 50/50 mix of antifreeze and water. Top that off with new heater hoses and you'll have a heater assembly that'll blow hot air like it's brand new.