I purchased my seat heaters from Cobra Heat and went with their waterproof version:
https://cobraheat.3dcartstores.com/
Can't address mounting them in the seats because I had them shipped to ERA and they installed them as they were building the seats.
Below is a photo showing the routing of the wiring. On the right you can see the wiring hanging down from a grommet and, on the left under the door sill, the wiring coming out of a grommet.
As for the switches, I mounted mine out of sight on the bottom flange of the dash. I lined them up with the outside edges of instruments so I know exactly where they are without seeing them. The third thing you see in the photo below on the left is a marine stainless steel accessory outlet:
Hope this helps.
Kevin