OP the simplest way to do this is like KeithGTP03 said. get a set of RCAs plug them into the back of the HU then run them under the plastic trim at the doors, once you get to th backseat run them under the back seat or around the side and then behind the plastic trim behind the rear seat back. All this cutting and splicing you want to do is going to take longer and cause you more headaches down the road.
In fact if you are close to Washington County PA bring it to me, I'll run them properly and you can sit in the house and stay warm. I would rather do it that way then have you take a chance at jacking something up. If the previous owners spent the time and money to do it right dont go in and start hacking away at what they did.
