The underhood hot water valve is either open or closed, depending on what mode the switch is on. The actual temperature control is based off of the cable.
The '79 A/C temp cable part p/n is 3842156.
On the case side of the cable, there is a weird adjusting clip that sometimes pops off. That Clip p/n is 6031132.
On no-A/C cars, the cable p/n is 3847103 for temp door, and 3847049 for mode door.
Note, the entire evaporator case (and firewall) changed from '76-79 to '80-89 and very little underdash A/C parts interchanges between the two year groups.
I hadn't seen any aftermarket sources for these cables - but plenty of good used ones and even a chance to find a NOS one somewhere.
BudW