There is an existing grommet towards the middle of the firewall, it's easier to find from the inside. Look in front of the center console and you should be able to find it. Poke a hole in there and then run your wire down the drivers side. The plastic panels are just held in with clips.

 

I used the rubber grommet on the driver's side of the engine bay (you'll see it by looking below the air intake pipe that connects to the throttle body). Simply drill or poke a hole the same size as the power wire and poke it through. The wire will end up coming out underneath the dash, in which case you can just run it underneath the kick plates and to the trunk.

Here's some photos:

 

Alright I just want to double check this is the right grommet. I took some pictures. This is it right?

I also had a question about securing or mounting the amp and sub. I will most likely be putting the amp in the trunk right next to the sub. How can I mount it to the car the so it doesnt fly all over the place when I hit bumps and stuff. I could screw it to the mat in the trunk but im afraid it would still bounce around.

 

Alright I just want to double check this is the right grommet. I took some pictures. This is it right?

Yep, that's the right place :thumbsup:. I recommend using a drill to make the hole. Choose a drill piece that's the same thickness as the wire so that everything will remain watertight. Use just the tip of the drill piece to make the hole, so that you don't accidentally cut into any of the loom in the process.

I also had a question about securing or mounting the amp and sub. I will most likely be putting the amp in the trunk right next to the sub. How can I mount it to the car the so it doesnt fly all over the place when I hit bumps and stuff. I could screw it to the mat in the trunk but im afraid it would still bounce around.

A lot of people secure the amp to the back of the rear seat. If you having folding rear seats then allow a bit of slack in the wiring so that the seats can still fold down. As for mounting the sub, I'm not too sure as I've never bothered with a sub in my Corolla. There should be a wooden cut-out that covers the spare tire. You could screw it to that. I'm sure someone else will be able to give you ideas.

 

Ok thanks for the help. Unfortunately I don't have folding seats in my car. I shall ask around trough. The only problem with screwing it to the wooden cut out is that the screws that came with the amp are very short. I could also get longer though.

 

On a model without folding seats you can't secure the amp to the seat because it's uneven, it's impossible to mount an amp to unless you put a panel on it or something.

There is a plastic barrier between the seat and trunk, I would remove that and cut out a piece of 1/2" MDF (make it larger than the plastic) then carpet and screw the board to the metal where the plastic was. Then mount the amps to the board.

Depending on your box, it may be easiest for you to just mount L-brackets to the back of the box. Especially if you have a folding rear seat. The panel above my spare is not secured, and neither is the carpet so securing the box to those would be a bad idea.

 

im planning to steal this idea from ricanrolla98 where he mounted his amp.
http://trdforums.com/forum/showthread.php?t=18315&page=7

as for the wire, you could pull it through on the drivers side lower fender. there's a gasket i believe where the main harness comes in behind the fuse box. well i know there's one on the 6th gen for sure but you will need to take off the fender. i'll have to check for sure if the 7th gen has the plug too. but the one in the center is mainly the ECU plug.