I have the same issue. Last year The push button on the "climate control module" quite working and nothing worked. The dealer replaced the module and the light started working again. I never had to use the Defrost mirror/rear window again.
Now this year, I have noticed that the button operates, the mirrors heat, but the rear defrost does not work. When i open the rear slider the light does not shut off on the module.
I did a little trouble shooting and power (14.85v) is getting to the rear glass. When i have the Rear Slider closed my meter says there is 517 Ohms, when it is open there is 0 ohms. I though that should be opposite. I seems like I have a continuety problem when the rear glass is closed, but the circuit must just bypass it when the window is open. It is telling me that there is a problem with the rear slider. Probably the contacts.
I am going to take it to the dealer again since I have been able to do my own troubleshooting, last time I was told it was working properly. It's pretty hard to back that up when there is frost on my window in the mornings that doesn't clear off.