for a starter I checked the Bentley manual on E 34. I thought this might help
The E34 (non-climate control) heater system consists of these parts:
a) Solenoid valves and auxiliary water pump at the left-rear of the engine bay.
Referenced respectively on EWD-131 at the upper-right, and EWD-133 at the lower-right. See my "Brake Fluid" procedure for a photo.
B) Heater cores (little radiators) with temperature sensors (installed behind the dashboard) one each for the left and right sides of the cabin. Referenced on EWD-131 at the lower-right.
c) The HVAC blower and speed control sub-system, referenced on EWD-132 at the top.
d) Control Panel, see the bottom of EWD-131 and 132.
e) Two stepper motors controlling fresh air and recirculation flaps, see the bottom of EWD-131 and 132. There may be more if the Bentley diagram is in error.
f) Integrated Climate Regulation Control Module, referenced on EWD-131 and 133.