The car will shut off non essential electrical components if there is too much load. A big gripe was the Heated Steering Wheel, which seems to be the first thing shut off.
You said that you put the seat on high and turned on the rear defrost. Sounds like that was just too much load at the time.
Generally, I would guess your battery might be a bit low. If you drove it for a bit and then turned on the heated seat, it would likely be OK.
What was the temperature? I had a pretty bad experience when it was -30° C here at Christmas and my car was parked outside for two days at my in-laws. The car started OK, but the battery was so cold that it wasn't charging at all, and I lost my clock, the actual heater stopped working, and various other things. It wasn't much fun! It was fine though when I got home because I park it in my insulated garage. Next time I will park it in the heated shop or at the very least plug in the battery tender. I didn't know just how sensitive the car actually was.
Batteries don't charge at all when it's really cold out. They need to warm up first, and with our batteries being in the trunk, they don't get the heat from the engine to warm them so it takes a while.