Who cares? It's your property. And yes, there are seats and there are heating elements already in the car. They even wired them up for you.
Authorities care. Yes it's your property and you can do whatever you want with it. The question is whether you're allowed to operate the vehicle on public roads afterwards. Cars go through thorough certification processes (homologation) and that includes software.
Can you prove that your change does not negatively affect a certification-relevant function?