i personally would have them relocate the power cable direct to the battery. the power wire is like a water hose, you are running a 4 agu wire to the back and expect 4 agu of power flow, you are currently tapped of a 8agu wire so the amount of power that is gonna flow is going to be equivalent to a 8agu wire. That is your bottle neck and you should go direct to the battery to ensure there is nothing along the way that is affecting how many amps you are getting to the back. It might not have any effect but i prefer to have a power direct from the battery to ensure no inteference from the fusebox or bottlenecks.