I'm thinking similar to 03SSLE, a possible circuit breaker. The symptoms you are describing kinda match what my Monte and wife's Impala did with the rear defroster (it might activate for a moment, just enough to show the grid lines and then stop).
I found it was a worn out circuit breaker. Replaced it and the defroster has been working great ever since. Not sure if the seats have separate breakers, but if they do, stands to reason that could be a possible issue.

 

yep mine still not fixed. lol

 

No, I haven't even started looking into it. I will at some point but it's not really a priority since I don't drive the car in the winter.

 

If you don't mind a newbie taking a stab at it... sounds like it might be something with the heated seat control module or thermister where the relay that turns that on is not staying latched. Perhaps the thermister thinks the seat is up to temp already and turns off the relay via the sense input to the control module. I've attached the wiring diagram for the driver side circuit for 2000-2005 (I suspect 2006+ is similar).

 

yes my passenger side is broken I need to replace the switch driver side works fine 05 Monte Carlo SS supercharged

 

yes my passenger side is broken I need to replace the switch driver side works fine 05 Monte Carlo SS supercharged
I have part number 6945-002 but they tell me it has to be an eight digit number

 

I'm having the same problem ten years later. Does anyone find a solution?