They do not have a wiring kit for that and they do not have the luxury of custom building one at a price they can offer that yuou could afford, the resitasnce issue is a cop out..go to the wire guage resistance table and see, 12 guage I think has an internal resistance of 1.56 ohms per 1000 feet...trust me..the resistance is not an issue..the fact that they don't have the kit and want to sell you an off the shelf one fit all is the driving point in their answer. Many upon many cars have their batteries in the trunk..however most aftermarket wiring is designed around an engine compartment install..my 66 car has the battery in the trunk from the factory..V8 etc etc..
For all practical purposes, build your own from donor cars...put you components where you want, easy access, easy replacement, use current model available parts that will be servicable years down the road..it is not hard..a wiring harness is just a bunch of individual circuits bundled and taped together breaking out at each componet for connection.