Is there a spare grommet, or something that I can use to pass a bundle of wires through the firewall?

Im trying to more permanent mount my AFX. I dont see where I can pass through my firewall.

 

On my 06 STR, passengers side wheel well, I have my fender liner removed. There is a wiring harness that runs from inside of the car to the under-hood wiring harness. This harness pases through a rubber type seal. This seal might be your best bet, it may have some room to play with running a few extra wires through it.