It looks like the previous owner thankfully cut an access point into the top of the gearbox housing to make it easier to access the switch. Image link below.
I assume the switch needs to be tightened to improve the connection, then lock in place with the locking nut. think I made the mistake of trying to do this while the car was in reverse gear with the ignition on (so that I can see when the reversing light is on). I ended up blowing the 5A fuse!
I will try again tomorrow. Since the bolt has the wires attached to the connectors, there is only a limited amount of turning I can do on the bolt without twisting the wires and putting stress on the connectors. Let me know if there is a better way to do this 😊.

https://flic.kr/p/2piWR11

Thanks.
David