Cars from the 1980’s rely on a very puny earth strap from the chassis to the gearbox. On our 1985 car, I added an extra earth cable room the bell housing to the chassis. It made a big difference. If the heater actuator cable is getting hot when you start the car it is a good indication of an earthing problem. Don’t grab hold of the heater cable to find out though - mine got hot enough to burn me.
