I just did this a few weeks ago. You can't visually see the grommet from the cabin. Open the engine hood and look for a group of wires entering the cabin through a rubber grommet on the passenger side.

I took a wire coat hanger and attached my amp wire to it. I then ran the amp wire into the cabin. Once the coat hanger penetrates the grommet you will be able to see it from inside the cabin.

Good luck.