I have recently retrofitted rear heated and electrically adjustable seats to a car that has never had any of this from the factory.
Here are some of my findings and pointers:
1. When buying seats make sure they have airbags in them if you have a FL car, if you have a prefl it doesn't matter so much.
2. Don't even attempt this without at least connector cut-offs of the loom, circuit diagram pdf's from the forum and some electrical knowledge
3. You will have to dismantle the whole interior, and by this I mean removing front and rear seats, center console and carpet to run the wiring the factory way which is like this:
4. If you picked electrically adjustable seats, make sure the headrests adjust and if not fix them before installing in the car.
5. Electric headrests are also supposed to retract at the press of a button above the head unit and there's a relay trigger wire for that. With a modern head unit with a reverse camera that's not required, so I have it wired to * of my driver seat memory switch, which also operates the passenger memory, so basically with one press of a button, you put the whole interior into its default position, so passenger seat moves to match driver seat position, and headrests move down.
6. New fuses need to be added for heat and electric movement. I used 2x 20A fuses (I used separate fuses for adjustment and head), the fuse holders are the same ones as for Bose, so i gut cut offs of these from Ebay and one of forum members
They look like this:
They live here:
And plug into the fuse distribution like this:
Lastly the seat heat adjusters:
They need constant 12v from the new extra fuse added
They need illumination wire connected so they light up at night,
ground
switched ignition to turn them on and more importantly off when you leave the car
they also receive temperature sensor feedback from seats and send the signal to actually heat the seats.
You will only need a new wooden trim and the heat adjusters, the plastic frame behind which sits under the rear vents already has the holes for them:
And lastly, don't cut corners, crimp and terminate wires properly, budget around 10-15h if you have bits off loom but not the whole thing, and around 8 if you have a complete loom, all labelled and out of the car.