This is the procedure I used on my 09 Corolla. I don't know if things are the same on the Camry, but I thought I would write it down here. I figured it couldn't hurt - you can always ignore it, but maybe there is something useful in here for you.
I used the rubber groment on the driver's side which had other wires running through it. It is hard to find.
Find a rubber gromet on the passenger side firewall which has nothing going thru it. It has two small cylinder shaped "nipples" on it. I found I couldn't really access the passenger side one so, I used the same grommet on the drivers side.
You have to use a flashlight and look down under some stuff sticking out and a little to the right of the engine. You identify the grommet by seeing a cylinderical rubber nipple. Try to mark approximate location with masking tape to help refind it.
It is really tough to find the gromet inside the car.
I cut off the end of the cylindrical rubber nipple on the engine side and put a straightened-out coat hanger through the hole in the nipple most of the way. With that done you can locate the hanger dangling under the dash to the right of the steering wheel. Then you can find the grommet by following the hanger up to the firewall gromet and mark with maskig tape.
The amp wire doesn't go through the cylindrical nipple. The amp wire goes through the unused space on the lower gromet. I think I removed the coat hanger to drill a hole close by. I used a 1/4" bit and electric drill to go through the gromet from inside the car. Be carefull not to nick any wires - I did not have any problems. Push the amp power cable through the drilled hole and feed wire through to engine side. Route wire to battery, install fuse and enclose wire with corregated vynal tubing for protection.