Yes, you can fit an aftermarket kit, such as this one from Waeco -
http://www.ebay.co.uk/itm/Land-Rover-Defender-Universal-Waeco-MSH60-Heated-Car-Seat-Kit-Driver-Passenger-/301612042233
Or, if you manage to find some genuine Land Rover heated seats, a wiring kit can be bought from here -...