grounding, chassis vs battery

No one mentioned that the true ground in a vehicle is not the battery negative but the Alternator negative - which in most cases is the alternator case.  The return path for your amplifiers is through the vehicle's chassis, the engine grounding strap, the engine block, to the alternator case where it mounts to the engine.  This is why the chassis is the best place to ground your system - plus big pieces of metal will simply carry more current than cables.  (The exception of course would be a system designed to be used with stand-alone batteries and the engine off.)

I am not a fan of running power cable outside a vehicle, even in conduit, as there are just too many things that can go wrong and the cable can be damaged.  I say run it through the firewall and through the interior of the vehicle.