Mine is under the carpet, but has been there for like 2+ years now... I pulled out the seats/carpet because i used to have multiple runs to the back and it would have been impossible to get them in with everything in place.
But for the most part you can install it will the entire interior in place, only thing is you'll more than likely have to pop off a couple trim panels to hide the wire.
Also can wire under the vehicle, but just need to be sure that you have enough covering to withstand heat and sum abuse from everyday driving.