couple options. I would run a 12 gauge hot from the battery or + terminal block with a fuse and relay at front run to the back and ground the heater locally in the trunk with a clean solid ground using 12 gauge. Voltage drop on single 12 gauge wire at 14 amps at 15 feet is about .35v if you start with 13V and ground it locally. Should keep the heat flowing and blowing.
