Dealers make it up as they go along.
A while ago, my heated mirrors didn't work on my Mrs Up GTI. Booked in under warranty. They suspected a BCM or wiring fault. The BCM would be covered, but wiring wasn't. I had to commit to an £800 strip down of the dash, before I knew if it would be covered. I declined...
A read of the manual, and tickle with OBDeleven and it was all sorted. No actual fault, they were coded to only come on with the rear heated screen! Yet the dealer hadn't checked that and wanted to bum me for the dash strip down hours...
I also had to re-arrange the fuses in the fuse box as they'd obviously been fiddling there and failed to replace them properly...
So, yeah dealers, clueless...