I know this is a common question but i need to get an 8 gauge power wire through my firewall of a 2000 Honda Accord Coupe. Here are pictures of the inside driver side and passenger side.

 

you should be able to locate a larger gromet on the upper driver side of the firewall. cut a hole in there and pull it through. if not, then you can pull it through the same as the main pull into your car. you shouldnt have any issues with going through the same gromet.

 

you need to pull back the factory barrier to see the metal firewall. the grommet is above the gas pedal. remove it, drill a hole sized for the power wire, then put the grommet back. then you can push the power wire through (from inside) and pull the length required to route to the battery/fuse holder. be sure to route it at this time. i use zip ties to hold the wire in place and I use split loom to make it look like factory and remain protected.

then you can use silicone to seal the power wire at the grommet so no water gets in. the silicone will need a few hours to cure, so do that after you've routed the power wire to the trunk and have your lengths nailed down.

 

the small grey wire to the right is the factory grommet:


for larger than 4awg, you need to drill. i found this location to be perfect:


i used the battery mount to hold the fuse holder. i had to massage it a bit and drill some holes (with it removed) to use nuts and bolts to hold the battery holder down.

 

in my car, I drilled that plate and the hole came out in the wheel well. I had to pull mine through higher up in the dash.