I've been building a replica '65 Cobra and was looking at all the aftermarket seat heater kits on amazon. This kit included all wiring, switches, and 4 heating pads which are twice the size of dorman pads for $29.99 delivered after 40% off instant savings when you click the "Buy 1, get 40% off" promotion button in the listing. Reviews are mixed, but that goes for most of the DIY seat heater kits on amazon.
Total for amazon prime is $29.99, was tax-free in Indiana.
Odd item to be looking for, but a really good price, next best price was $46.
If you install these, you need to make sure you place them right under the upholstery, not under the seat foam (or you will be disappointed). They draw 5 amps per 2x pad or per seat and do not include main power relay, so be prepared to add that unless you have a spare 12amp circuit in your ride.