reverse sensors not working after towing

hi, used a mates 4motion transporter 2007 with what i think is a hayman reese towbar and when i returned it the reverse sensors no longer worked. I have found in one thread here that they made the sensors disable when trailer plugged in but would expect them to work again when the trailer plug is disconnected. Any tips?? I belive the fuse is no.8 in the lower fuse holder under the dash and that seems fine.

The little micro switch in the trailer plug isn't jammed in, is it?

If not jammed within the car's trailer plug, I tried this after the same problem, switched off vehicle, got out, locked it with remote, unlocked it, restarted van and put it into reverse (auto in my case ) and it started beeping again. Don't know why it solved the problem.