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Are the hoses leaking coolant? Is the small pump located on the engine or some where near the firewall?
I think what you are talking about is the auxiliary water pump or coolant recirculation pump. This pump improves coolant flow through the heater core at low rpm.
Is this what you are talking about?
There is also the heater core shut off valve or "mono valve".
I found a clear picture of a 78 model
The heater servo is circled in red, the auxiliary pump is circled in blue.
Let me know if you need anything else before you decide to rate me, thanks!