We took it apart and found a PCB with a lot of discrete components (transistors, resistors, ...) and a single IC, the PT8A978. After a while and random guessing we figured out that the IC could be related to its "brother", the PT8A977 for which the data-sheets are available. In short, the following pins are input to the amplification circuit:
- pin 6: right
- pin 7: left
- pin 10: backward
- pin 11: forward
By disconnecting these lines from the IC we can directly connect the amplification circuit to a BTnode (or any other microcontroller) and don't need to build our interface.