What I found was the Porsche diagrams (Service Information booklet, 1978, pgs 130-131) were very inaccurate and lacking info. I'd suggest not using them.
Now I understand why early car owners have struggled to get their HVAC system functioning properly (after what I went thru, I'd say near impossible).
The following information should help you get your system working properly.
This should apply to both MY78 and 79 (however, I wasn't able to test a 79, so YMMV)