It does indeed seem to be aftermarket seat heater, after finding this posting on North American Motoring's site (titled Aftermarket Heated Seat Kit Install 2008 MCS
(R56)). The switch in my Mini is exactly the same. I couldn't post the link because I just joined Mini2.
It just seemed a little odd that it was only for the driver's seat and not both, since the article mentions they kits are usually sold as pairs. Maybe the previous owner had too much of a battle installing the first one and decided no to install the passenger side heater.
I also found that the previous owned had installed an oil pan heater and a battery warmer. These kind of things are very Alaskan necessities.