This has just started happening to my 15' XL ~maybe 2 weeks ago after the cold and snowing started.. Was not an issue this summer/fall. The rear defroster works damn well too and clears snow and ice in a few minutes on mine but i'll go ahead and clean the contacts and cake them with dielectric grease to see if that makes a difference.
One thing to note is that it's also having the loss of GPS, NAVI, XM issue related to the satellite antenna allowing water in causing parasitic battery drain but has yet to leave me stranded. The battery is noticeably weaker since the GPS/Navi/XM started failing. Not sure if anyone else was having that issue in tandem.
I'll report back after testing the rear defroster contact R&R