Are you going to be running the wire from the switch inside the cab to a relay under the hood that controls your lights?
I wouldn't recommend running the power inside the truck to the switch then back out to the lights directly... there will be a high current passing through the firewall which isn't a good idea, plus it's never a good idea to run lights directly from a switch without using a relay.
Using this diagram as a guide, you can run a trigger wire from the switch to the relay under the hood. This keeps all the high current wiring under the hood & will lessen the chance of anything shorting out or catching fire.
With all that said, you should be able to find a factory grommet in the firewall that you can push your wire through safely. ALWAYS use a grommet when running any wire through the firewall.