You should be able to drill a hole threw the rubber part. I've never seen the firewall on a f-150 though. so im just telling you want i've done to my car now and the 1 i used to have.
Look carefully at the firewall. There's always some hidden. They will never make a firewall without holes. Wipe away the dirt. They're usually black rubbery plugs. Leave the drilling as your last resort cause if not done properly, you could induce rusting.
OK, I hooked everything up, and guess what??? IT WORKS!!!! I love the deeper sound and how it fills my truck now, but what a nightmare running wires...