Basically, here's my plan. Get seats from a junkyard. Strip off outer cover. Shave foam fron seats and backs to insert massaging/heat pad (I'd have to shave the foam because if I left it in the I'd have to sew new covers and the seat bottom would sit too high). Recover seats. The electrical input for my pads are 12 volts DC and one came with a cigarette lighter adapter. So power's no problem. I just need to dismantle the control head and mount it into my center console and I figure its done.
I've had unmodified pads strapped to my car's seats for a couple winters with power from the cigarette lighter and it worked great except they don't get hot enough nor quickly enough - compared to the OEM heaters in my Caddy. I lucked out with the massager having a built-in timer of 15 minutes (its not recommended to run it longer says my masseuse) and it has the magnets in it for increasing bloodflow. Mine have five massage sections: neck, shoulder, kidney, lower back, and thigh with heat in each section which if it would get as warm as OEM would be a significant improvement.
Good luck with your attempt.