Went to the local 4WD centre this morning. Apparently the earth on the plug is not connected / faulty. Let the dealer know about the issue straight away... he'll be arranging the fix on Monday. Not happy
Well, got it fixed yesterday. Took an hour only - apparently they have installed the tow bar 1st, then proceeded with the reverse sensor installation during which they have dosconnected the plug and didn't connect it back. Not good; however the way the dealt with the problem almost compensated the troubles - e.g. same day, time suitable for me, replacement car (for an hour), etc