It is my understanding (through testing) that you only need a good ground and need not run it back to the battery. (someone will correct me if i am wrong im sure)
What I did was ..... removed a BIG screw that held the back seats in place and grinded the paint off the screw and the area around the hole the screw gets secured to.
Then I tested the conductivity of the connection and it worked fine.
It was alot easier this way ..because now I only have 1 wire running from the bat and the other is only about 12 inches long from the screw already in the trunk to the ground.
Hope that helps