I don't recommend any over the other but this is what EBay came up with.
eBay - heated seat kit, Parts Accessories, eBay Motors items on eBay.com
As a former OEM seat designer I've worked with a few heated seat manufacturers. We made heated seats for the Jeep Grand Cherokee and Acura. Our's were actually sewn into the seat. Heater construction is pretty simple. Probably the hardest part of installing an aftermarket kit would be the electrical. You would definately want to install a relay so that it can only be turned on if the key is at least in the ACC position. See how many amps it draws and install a relay that is sized appropriately. Add Add-A-Circuit is an easy way to tap into a fuse box that doesn't have any open spots. I did an aftermarket power window install and this is the part of the write up about those two items.
1- 30 amp relay - $25 (included shipping) Purchased at
A1 Electric Automotive Accessories, Power windows, Power door locks, keyless entry systems, Window motors, Window regulators, Pop door kits (which also sells the kits but much more expensive than they can be had on Ebay but Ebay sellers don't offer a warranty). Part number RELAY1. Next time I would make my own from parts purchased at my local autoparts store. Would cost about $15.
http://i14.photobucket.com/albums/a3...ndw/relay1.jpg
1- Add-A-Circuit - $15 purchased at my local Napa made by LittelFuse part number FHA200BP. A super easy way to provide the on ignition power to trip the relay.
http://i14.photobucket.com/albums/a3...rwndw/fuse.jpg
heated seat kit? 
Linear Mode
