You are right, you need a wiring diagram. I don't have one to look at as I am at home but I would look into the defrost positions control of the blower controller. The defrost is always at high speed.
I would also look at the blower resistor schematic and see if they were still using the overheat bypass switch like earlier cars. This was a switch which shunted the resistor coils if they were getting to hot. The blower would go full speed and the coils would cool. Don't remember ever seeing that fault on a 123 body.
Bosch Master, ASE Master, L1
33 years MB technician