03 Camry Firewall

Where in the firewall can i run the main power wire for an amp install on an 03Camry?

well you can look for a gromet or drill a hole I would suggest on the side that the battery is in for a shorter cable run. if there is no grommet than break out the drill start with a small bit 1/8th is what I use (or a uni-bit ) as your pilot hole and then the size that you need just check twice before you drill a lot of times the brake lines run down the fire wall. Use a punch to mark wherer you want to drill from the inside of the car and look through the hood.  for the mark that way if you are in a bad spot you know before you drill..

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don't fowl the track

Find the wiring harness that runs in to the car from the engine bay and poke a bit of stiff wire into it, check inside for wire. If this doesnt work try drilling a small hole above the accelerator cable. this is done from the inside under the dash.

I've only drilled a hole to run power once.  Usually I find a body plug somewhere, and cut a small "X" into it.  If all else fails, you can drill, or (some may frown upon this) cut a slit in the boot the steering shaft enters.  If you cut this boot, ensure that you route the cable away from any moving parts (use some zip ties if you need to.  And make sure you seal it up with some silicone.  Usually I never need to do this, you can usally find a plug some where.

Oh yeah, it's common sense, but if you drill a hole, be sure to use a grommet.