ALWR (A6,4B '98): Engine Control Unit Problems

I have transplanted a lightweight steering system from an A6 4B into my convertible, which was manufactured in 1999 (model year 2000). (The entire system fits together very well in terms of gear ratios.)

Installed, tested, and calibrated. The diagnostic function works perfectly; the upper and lower positions are easily reached. The set position is accurately reproduced.
The control unit is coded for static headlight range control/sedan.
The level sensors provide plausible values (I still have an A6, so I can compare them very well).

So far, so good...

I am now experiencing the following problem.

I turn on the lights, and the headlights move exactly to where they should. After about 5 seconds, they move all the way down (even below the "down" position) and stay there. I have NO error messages in the control unit!
When you turn the headlights on and off with just the regular light switch, they remain off. It's only after turning off the ignition that the problem starts again.

(I connected the power supply of the control unit to terminal 15, but I'm not sure if it actually needs to be connected to terminal 30...)

Best regards from the Ore Mountains.