Retrofitting of heated mats is AFAIK not so simple: if one attaches them under the upholstery the heat transfer is strongly weakened (perhaps someone can report here from personal experience).
Have you also considered heated jackets? They also warm the arms and neck.
I have used a heated vest so far. Now I'm upgrading to a heated jacket (
Keis) because I also want warm arms (without side windows my left arm [LHD] always gets unpleasantly cold).
The wiring is simple because the battery is just behind the seats.
