Would you like an educated guess? There's a wire to wire short in electrical connection at the transmission reverse switch.
Do you want to know for sure? Test! With a test light confirm there is key on power at the transmission reverse switch OG wire. Now test for power at the BK/PK wire. If there's power at the BK/PK wire this almost confirms the diagnosis.
If this were my car I would look closely at the connector itself as the source of the problem.