Before clogging up the underdash area with the heater box, I put the new vents and defrost duct hoses in. The shorter of the two goes on the passenger side, and it seemed easier to mate the vent and duct outside of the dash with a zip-tie for good measure. | 
A new seal goes on the interior heater box where it mounts to the firewall; again, a little weatherstrip adhesive is a good idea. |
This view is from the passenger seat looking through the glovebox compartment. Prior to reinstalling the interior heater box, disconnect the cable that goes through it and into the firewall and connect it to the heater core box. There is a little removable rectangular plastic plug on the interior box that you will pull the cable back through inside the truck. I also lubed the cables to help them push and pull as smooth as possible after all these years. | 
According to my marks, I reattached the cables to the heater controls. Be sure to use the little star keepers on the cable pins to prevent the cables from coming off. |
With the heating system back in place, here's another shot through the glovebox. The heater box looks good thanks to the clean-up and paint-the new ducts are routed, the cables are hooked up, and the wiring is reconnected. | 
Check it out! Even though it's a small detail, the controls look like a million bucks, and they work to boot! Now I just need to replace the old heater hoses and fill the radiator with clean coolant. |
The '72's glovebox was nonexistent when I got it, so while on the phone with Harmon's, I ordered a new one and its mounting screws as a fitting finish to the project since I won't need to access the underdash from there now. | 
Slowly but surely, the C-10 is transforming into a more useable and cool pick 'em-up. Call Harmon's today for your Chevy truck needs! |