[JSV798]

I have a '78SC and have worked on this control unit at various times over the past 22 years. The part you show has the servo in it that operates the flapper boxes underneath to allow hot air to pass into the interior ( my car has levers as well by the handbrake so if the unit fails I can still open the flapper valves and get heat or I will be able to when I fit my newly purchased heat exchangers). It also switches on the fan in the engine compartment and that's what those two spare brown wires do. You should find two yellow wires under the carpet between the seat belt anchors.
The PO had messed around with the heating system; the ducting and fan in the engine compartment were missing as was the thermostat between the sun visors.
As someone remarked that thermostat is NLA but I did eventually get a used one from a breakers so that would be your best bet.
The best I have ever had my system working was that it would switch the fan on and open the flapper boxes; then the centre control unit stopped working so I just put a switch on the two yellow cables that emerge from under the carpet (and connect to those two brown wires) so that I could switch the fan on and used the hand lever to open the flapper boxes.
I bought a used control for about $30 and that worked for a while but that has now failed. Recently tried to switch parts between the two units with limited success.