Increasing the wire length +10ft would effect your values, primary wiring is usually anchored every 1ft. Go to a smaller gauge (larger size) wire and use your DVOM (digital volt/ohm meter) to check the ampereage, and voltage drops from front to back. The fuse boxes are in the engine bay for a few reasons though, the wires have less chance for corrosion over a smaller length, also chances for excessive voltage drop (over .1 volts) decrease with a shorter wire, wire length can change resistance values. Also, you need to establish good grounds in the trunk, make sure you ground to the chassis and not to some part of the trunk compartment. hope I helped
-Dave.