Diagnostic plug for old 1Z engine

Hello everyone,

I own an Audi 80 1.9 TDI, 1Z, manufactured in 03/94. The diagnostic interface is located in the control panel and consists of the two 2-pole sockets (K and L / +12V and GND).

About 2-3 years ago, I built a circuit board, but it never worked. I still don't know where the mistake was.

Now I would like to try a second attempt, using a simple but functional schematic. I've looked at those things and have to say that the "Worst-Case Design 4.0" looks quite complex, but it must also work perfectly. "The 'minimal version' is definitely too simple, since it didn't work even then (I think it was the schematic from Mark that I tried - but I don't rule out the possibility that I made a soldering or wiring error)."

Is it worth building the "Worst-Case" circuit? It is relatively flexible due to the KL/KK switch and also shows everything well through the LEDs. Is there a Conrad parts list (or similar) for it? Electric retailers)?

Thank you!