Weird towbar electrics.
 

Hi, I've just recently bought a pre owned xc60 and had a towbar fitted with 13 pin electrics. when i got the car back, being the curios person i am i checked the wiring with the mot test equipment from work, surprised to find the fog or reverse lights showed up as not working, checked with a 21watt bulb and no feed to the socket, returned car to towbar fitters and told them the situation, when they plugged there test equipment in everything was fine. they told me the all electrics need to be plugged in for fog and reverse to work, so travelled to caravan storage and plugged in, everything ok.
they said they get alot of vehicles in that have been mot tested with a Led test equipment and failed but nothing wrong and also you cant check fog or reverse light feeds separately with a bulb,
Has anybody come across this. Car electrics are now baffling,

You've got CANBUS and other magic trickery to thank for this!