i believe it just needs to be tapped into speaker wire somewhere along the line
basically it just takes the signal sent from the radio and puts it to the RCA's to send to the sub amp
i dont like messin around with the wire in the doors, besides any normal amp wire kit will give you a set of rca cables long enough to reach to the dash, so thats the other reason i would put it there, so you dont have 15 feet of cable wrapped up somewhere in the rear
but being that i've never done this on our trucks, idk exactly how much room is behind the radio for that to fit, i'd imagine a decent bit just because i can fit the bose harness behind there and that thing is huge