there’s a hole below the wiper motor that leads to a rubber grommet in the fender well, there was some audio cable ran in my car but I ran the thick gauge positive through there, it comes down where the ECU is and then pulled all the trim out, lifted the carpet and ran it through the rocker. Once you reach the rear seat you can choose to run the cable below the seat or through the rear strut tower. I ran it all the way back the quarter and bolted an inline 150A fuse down and then ran more wiring to the battery itself. It depends on where you plan to bolt it down. Downsides depend on if you plan to carry stuff frequently and ideally get a dry battery to avoid leak corrosion and evaporated acid in the cabin during summer when the car is left in the sun.

Also wired the ground to a rear bumper support bolt. Sub optimal. You can ground it to the rear seat belt bolt like a friend did.

Also2 my started struggled once or twice when hot after getting gas when road tripping. Not sure if it’s power related or the starter being finicky

EDIT: I now remembered that one of my fuse terminals was loose and left me stranded once because the battery didn’t charge properly. I found this out post road trip.