1977 first does not stop running - then does not start.
 

My 1977 930 does not start currently. The following is confirmed:
- In the weeks leading up to not starting it repeatedly did not stopping running despite the ignition being turned off - that is also what happened the last time it was turned off before it did not start. Usually after a few tries it would turn off.
- It is not the ignition - I connected a new ignition switch to the connector to no avail
- The car starts and runs fine when shorting the magnetic starter switch

I think it is highly likely that the faulty behavior turning on- and off is due to the same issue. Insights would be much appreciated...

Common fault in the Turbo. Search for run on timer relay.

I am struggling to find a 77 wiring diagram to verify ... If I understand you correctly you see the starting problem (no power to the starter) potentially caused by the time delay relay for ignition cutoff?

define it does not start.
does it not turn over?
or does it not run?

I assume it does not turn over.
does it have an aftermarket alarm system? if so that is probably the first place I would start.
check and clean all the power connections and grounds. tranny to body, battery to body, terminals and have the battery tested.

does the starter make a clicking sound?
use a test light.
compare the battery terminal at the starter to the yellow wire when cranking (trying). the light should be the same for both. a MM will not work. it will measure 12v and there not be enough power to pull in the solenoid.
if the light is good, your starter is bad, assuming you have a good batt and cleaned the connections.

as far as it will not cut off, with the problem above and this one it sure sounds like bad sw.
even if the delayed action relay was sticking (power for CD), the FP;s would have to stay on for the engine to keep running. only thing I can think of that would keep both energized is the ign sw. unless again you have an aftermarket alarm and maybe they tied the FP's and ign into the alarm. no usual though.

No, not the same thing. You had said that the car did not switch off. As the Turbo is designed to run for a few seconds after switching the ignition off, the fault can be caused by the run on timer circuit not working properly.

If the car is failing to crank, that is a bit different so I'd agree with T77.

Thanks for your help! Short update:

1) Starting problem: Turns out the cable from the ignition to the starter runs through the front trunk and a connector had come undone!

https://imgur.com/l0vzNmZ
https://imgur.com/odwkfER

2) Stopping problem: I am waiting for a replacement Ignition Cut-Off Relay (93061711702). Though I did figure out that the care does not stop - unless the BREAK pedal is pushed. No idea if this is the itended way of working?
Edit: same behaviour also with new cut off relay....