routing pwr under rear seat

i have an 07 gmc sierra crew (new body style). what is the best way to route pwr from the battery under the hood to an amp that will be mounted under rear seat on driver side.  multiple options are greatly appreciated.

If you don't have experience with this stuff, then I suggest having it professionally installed.  They will know how to run the wires cleanly, as well as properly get through the firewall.  And if it is a reputable shop the results will be a very clean install.

i do have experience running wires, just was asking  for options before actually looking at the task at hand

Oops.. didn't see that you wanted under the driver side rear seat. If that's the case.. just run it under the panel/carpet on the driver's side all the way to the rear seat then underneath the carpet..

where is the firewall opening located exactly

Odds are you will have to make one. Best bet is to look up at the firewall from the drivers side inside the truck and see if there are any openings that have some kind of plug or cap. If not, you will have to drill a hole to make one, but you have to make sure there is nothing on either side of the firewall that would be hit by drilling there. This is what I did in my vehicle. I just inspected the firewall from inside the vehicle and under the hood, I had an empty space close to where a large wiring harness went through the firewall and put mine there. You will want to get a rubber grommet that will fit your wire and drill the hole size accordingly. Then just run your wire through. Hint, pulling wire through tight places like that, works better with two people.