Stay away from that dealer at all costs.
If your camera is no longer communicating with the head unit and it truly is a 'wiring harness' issue (as the dealer claims), then I'm 98% certain the problem would be right at the area where the yellow tape is around the wire bundle going up into the trunk hinge arm.
View attachment 30745
Every time the trunk is opened, that portion of the wiring bundle is straightened out and then returned to that bent position when it's closed. That could eventually break a wire.
Any good automotive electrical specialty shop should be able to splice in a foot of new wires in that bundle in a few hours. Toughest part of this job is getting the trunk panels out. I would splice all wires in that harness since you'd have it opened up.
Did the camera just stop working altogether or did it start acting up and then finally quit for good? Chances are a fraying wire would have given you a couple warnings before quitting completely. If it did just one day quit, I'd suspect the camera as a possibility. I see one site online that has it for $262. Also, it's much easier to get to, only having to remove the trunk lid liner and a few screws.
Sad to think your dealer would rather stick you for a full wiring harness then recommend a much cheaper alternative.