Ok I will try and explain this as best I can if I'm wrong I stand corrected and I will have learnt something new today
As I said before the Lenz is a 5 function decoder
1st function is F0 side lights
2nd is F1 red tail lights on and off
3rd is F2 head lights these are directional
4th is F3 shunting speed
5th is F5 cab light #1 end
The reason f5 works the cab light is because that is probably the factory default. You could go into the CV settings and remap F5 to button 4. To get the cab light working on the other side I would assume you would need a six function decoder. The cheapest 21 pin 6 function is the TCS DECODER