C220D problem help please

[sone]

I have borrowed a trailer from a friend and have been messing about with the lights on the trailer as its a bit worse for wear. Anyway when I entually removed the trailer I have a constant high pitch noise coming from the back of the car which I can't stop unless I remove fuse no9.
If i remove the fuse I have lost the interior lights and also I've lost the ability to lock the doors remotely if the fuse is in or not!.
I've checked all the other fuses and now I'm lost for ideas, anybody got any ideas.

Cheers

Steve