AUY Control Unit: Sharan Power Supply & Issues

Hello everyone. Maybe someone has a tip for me on how to narrow down the source of the error.

The Sharan, manufactured in 2002, simply stopped running while driving and won't start anymore. The control unit could no longer be accessed via the diagnostic tool. Relay 109 was only clicking rapidly with the ignition on. The radiator fan would occasionally activate, but always for just half a second and completely randomly.

I then replaced the fuse labeled '109,' but that didn't work. I also tried bypassing it, but still no success. When the '109' fuse was bypassed, I measured the power supply at the control unit (STG), and everything seemed to be present.

And by chance, I discovered something: As soon as you ground the housing of the MSG (Multi-Station Gear), everything starts working again immediately! The motor starts! If you disconnect the housing from ground, the motor stops.

I then opened the cable harness slightly, so that the battery ground connection is present at the pins. What could cause such a fault?

Hello!

I've spent the rest of the day troubleshooting this issue and I found the problem. While the power supply to the control unit (STG) was fine, both pins for the ground connection were broken off just before the circuit board! For some reason... And it seems like the STG needs a substantial ground connection, as there are two 2.5 square millimeter cables running to it; I think it's necessary for controlling the power distribution (PD) elements.

I have now resoldered the contacts, and everything is working as usual again.