I made the whole install in my garden - so plenty of ventillation. But the car was standing with open windows for three days to get rid of the smell.
I bought the seat heating pad from aliexpress - the cheapest unit, without any wiring harness or switch.
i also bought a rocker switch with yellow LED - to match the factory switch for the driver seat.
Finally, I bought the cheapest 12v thermostat circuit also from aliexpress.
ooo, and also bought a fuse separator (rated for 40A), guess... yes, from Aliexpress.
Total cost was less than 30 EUr, incl. shippment.
The wiring: I used the fuse separator to separate the cigarette lighter fuse as it is off when ignition is off and we never use it, anyway. The initial fuse there is 15A, I changed it to two 10A fuses.
Negative pole I found under the fuse box - a quite large screw is there.
i installed the rocker switch under the center console, next to the USB charging port - I guess that is the factory place for it, but it is hardly visible from the passenger seat, so not a great location.
As there is a large whole behind the backrest of the seat (you need to open the zippers on the sides to gain access to it), I decided to put the thermostat there. I set it to 38 degrees as I read on the net that it is the recommended temperature for adults as being the lowest temp the body feels warm and muscles start to relax. As I like the simplicity of the car dashboard, this way I managed the setup that there is just a rocket switch on the dashboard, but you actually get thermostat controlled seat heating.
My next project will be the heated steering wheel (we have it in the Leaf and it is very comfortable) and I am also in the process of getting 12v infrared heating film which I am going to install under the floor carpet in the front and behind the door panels.