I have never seen a "check connector" in use and the only thing that I would imagine it could be used for, would be a device similar to what was used by Nissan and called a "Detector Drive Counter" which in that case monitored the distance the vehicle had travelled and indicated when the EGR control system should be serviced. Just guessing here but possibly the harnesses were built to spec for a similar purpose, but the devices were never used.

Edit-Posted before I read your post Tomo. Can you elaborate on that a bit?

AFAICT, the BCDD diagnostric connector is to check if the BCDD solenoid has 12 Volts, and the speed switch on the speedometer is working.

Remember, these engine sytems aren't as sophisticated as the current ones, so all they care about is the air & coolant temp, and the throttle setting & vehicle speed. All just to keep the air-fuel mix from getting too rich at any time and spewing gasoline fumes out the tailpipe.

what's the average price of this harness? i might just get another one instead of trying to repair this one

Just picked up a 77 280 z and found a spare harness inside the car, overall appears to be in good condition I,m guessing it to be from a manual trans car,if this will work for you it,s available.