Maybe you can explain in more detail what trouble you are running into?

 

Just when I run 0 gauge plus all the other wires, I have a hard time time running the wires from the battery to the rear of the car in a way that the carpet and panels can go back together super well, depending on which vehicle I am using so I’m wondering if I should open myself up to the idea of running wires underneath the vehicle?

 

You should have room down the passenger side for your power wire and everything else can be run on the driverside or down the middle. Might require more disassembly than you would like, but that's how it be sometimes.

 

Sometimes running down the centre along the transmission tunnel (inside the car) might be eaiser.

If I was running under the car, follow a factory loom and I'd use proper abrasion rated double sheathed welding cable, correct standard escapes my mind.

 

Thanks for the input. I have recently become a fan of utilizing welding cable for power/ground, only downside being flexibility

 

If running cables under the cabin, you have to seal all cable passes, avoid heat from the exhaust system, stay well away from moving parts like drive shaft and suspension, make sure there's enough room for all maintenance work and route them safely so there's no way a rock or bump could damage it, or even that an accident could provoke a short circuit, sparks, etc, therefore away from the gas tank.

That's why most people always route all cables inside the cabin no matter how tight or difficult it gets.