I cut out the seat elements out of a 91 jetta and put them under the upholstery of my 84 Nissan pick up. I used a dash light rheostat to control the voltage going into the seat to set the temp just right worked great for the 10 or so years I had it in the truck until I sold it . I used the rheostat because I did not take all the other relays and wiring stuff from the scrap car.
My 83 Caddy I swapped in the seats from another 91 jetta and used all the factory stuff and it works just as well .
My wife bought a 2014 jetta and the heated seats are crap , you have to turn them on every time you get in the car , it only has 3 settings and high is to hot and med is to cold and the difference between the 2 sides is quit different
That's my thoughts on it.