Seat heater are a funny thing to include. No need to get into any software changes, could just install an alternate switch and power supply to get them working.
i'm sure it can be done, but not as simple as you'd think.
you cannot change the 12V battery on a BMW without then coding the battery to the car. if you don't code the new battery into the car the car won't work.
reason? "we need to make sure the battery you install has sufficient quality Ah"
these things are all hard coded.
makes sense now why BMW were pushing so hard for their new "iDrive 8" bolt on screen and removing all buttons.