I'm sure the answers are here, probably multiple times but I seem to be struggling to find the magic combination of search terms that gets me relevant results.

I am pulling my JBL system and replacing with aftermarket components. I would like to run power from the battery to the amps - one under passenger seat and the other in RR quarter panel (same spot as factory JBL amp). What is the best way to route these wires? I assume I'd want to cross the firewall and enter on the passenger side? I see an easily accessible grommet with an extra nipple I could cut and use, but there's too much crap under the dash to see where exactly that enters.

I suspect there's probably an easy answer, I'm just thinking of the old "measure twice cut once" rule and applying here. I really don't want to cut anything until I'm sure it's the best location to use.