Mercedes Atego

Hi, everyone, new here to day so hello to you all.

I have a 04 plate Mercedes Atego 7.5 ton 815 and the reverse lights are not working. I have checked everything with a meter and the problem seems to be that from the gearbox switch the wire goes back into the cab and well… where do I go from there? The lights work the switch works all the fuses are ok. I am thinking there might be a module or relay unit some where. I also have no wiring diagram. Any advice or information would be great.

Cheers.

Paul