tricky 300D A/C heating question

[tschepplmasta]

Hi! Finally, I got the A/C of my 300D (1980) fixed , but one problem remains:
When I set the temperature dial to cooling, everything is perfectly working and normal. But if I try to heat, the temperature of the air coming out of the passenger's nozzles is much higher than on the driver's side! The same is true for the foot air outlets. Why is the car heating properly only on one side? Does anybody have an idea?
Help would be greatly apprechiated!
Thanx in advance,
Tschepplmasta.