On the 78 at least, there are two magnet valves located near the right shock tower, just ahead of where the infamous fusable links are. Because of engine heat, etc. the vacuum hoses to these valves tend to crack. When they leak vacuum there is not enough to open or keep open the vacuum cock that allows the heated coolant through to the heater core. Thus, no heat. There are two water cocks in the system in the upper left passenger footwell. One is manually controlled by the heater levers, the other is vacuum controlled by the vacuum selector valve. That's the best I can expain it. You might want to consult a service manual.
Good luck,
Mike