No power to reversing light. I've checked the fuse and checked the gearbox switch. There is 12v at the fuse but no power to the gearbox switch. Is there a relay in the system? Any other suggestions or does anyone have a wiring diagram? Thanks

Interesting,,, as I have a dud switch on mine. Also a reverse camera switched from the same circuit that I will be messing with to upgrade the camera.
Download the relevant eLearn manuals off here, there is a circuit diagram in them. They don't always download correctly, do them 1 at a time. I use Gburner virtual drive to make them into an .iso file, then mount it as a virtual drive with gburner, and open the manuals that way. Bit of a faff, but easy once you have done it a time or 2. Check download link in blue header bar near top of homepage 3rd item from left

Be aware, that AFAIK, these vehicles use 'earth side switching', the supply + being present at the device constantly, being given its swiching 'on' by connection to earth (as opposed to being already earthed and + supply being controlled by the switch; effectively, 'the other way round' if u see what I mean!) If you get the diagram, I think anything with a D prefix is a connector, and likely source of mischief.

Likely scenario, is a corroded connector or wire, of other dirty/crusty /broken connection under bonnet.

There is an intermediary connector about 400mm from the switch undey the air leaner Are check that for continuity / corrosion

Depends what chassis you have (Fiat or AlKo). My AlKo chassis 2003 Burstner had a multiplug a few feet forward of the back end on the UK nearside. It is liable to getting mucked up and wet where it is situated.