Kevin:
Like you, I have a 2006 DTS. It was snowing this evening when my wife and I went out. When we came out of the theater, the car was covered with snow. I cleaned off the snow on my windshield and side windows and not the rear window. When I got back into the car my wife asked why I had not cleared the rear window. When I explained, she reluctantly agreed to get out of the warm car and into the cold snow and wind to make sure that I did not back into anything. Long story short, the rear defroster melted all the snow off the rear window except for a strip about three to four inches wide at the top of the rear window. My wife said that it appeared that the top four wires did not do any heating. It appears that your dealer was right. I hope that this information is helpful.