I bought this kit, had it fitted by local upholsterer and connected by auto electrician. Cost about £500 all in. Very impressed with it - as good as any of the factory fitted heated seats I've had before.
THE KIT INCLUDES THE HEATER PADS FOR THE SEAT & BACKREST FOR BOTH DRIVER & PASSENGER SEATS, WIRING, SWITCHES & ELECTRICAL RELAYS FOR A PROFESSIONAL INSTALLATION. Once installed you will have a very professional looking heated seat system.
www.ebay.co.uk
Waeco do cheaper versions too but after a lot of research, this is apparently the one to go for.