I know I will be bashed for this but other people have asked about it but you could connect a switch to turn the subs on and off. I would not necessarily recommend this to you because it doesn't sound like you know a ton on electrical wiring (no offense). And with the whole in line converter I would personally put it in right behind the head unit and run the rcas from the front of the car to the trunk. And as for the gauge of wire you need you need to first need to know how much watts the amp needs and then calculate out how big of a fuse to get for your wire. If you have any questions feel free to pm me.
