Hi Thomas
After my search I found that your model (BR50 with tub tender (Wannetender) with DB markings was released from 1987 and replaced shortly thereafter by Delta Digital models
On a German website I found an extensive study of electronic reverse unit and there were multiple types.
The German web site is The good old electronic inverter unit

The unit likely to be yours looks like this


There are 2 wirings possibilities
- a full version with low voltage lamps and direction dependent lighting (Your case I think and a simplified version with standard lights (16 V) permanently lighted

The full version is this


I hope this helps
Jean