I experienced this issue just this weekend. i was driving from NJ to Chicago and noticed this happened after a couple hours of resting in the car at a rest stop. After I got to my destination and for the remainder of the trip I have not experienced it again (but i suspect it will come back again).
I did have a USB cigi charger powering a device and I also did turn on the seat warmers immediately after starting the motor. The issue was only with the drivers side seat warmer button on any of the 3 temperature settings. Basically, it would blink off for a few seconds and then light up at the desired settings 9and would keep cycling On/Off. No aftermarket accessories are installed in the car at all (I am FBO, but that shouldn't impact the seat warmers). i am on the original car battery. 2008 335xi @ 58k miles