Lexus cars that came from the factory without the heated seat option use a different wiring harness that does not have the necessary plugs for this operation. To do this you'll need, the proper harness, new/used seats that include the heater elements, switches, relays, and anything else that is normally used on cars so equipped. Very expensive conversion!!!
My suggestion to you is to buy aftermarket heated seat pads that'll plug in to your cigarette lighter. Much easier and cheaper. :)