The hole used for the upfiter switches in the grommet for the main wiring harness through the firewall should be enough for any sort of aftermarket wiring needs. You can use relays external of the cab. If you don't know how to wire relays there are plenty of diagrams available and several aftermarket switch / relay kits that can be purchased that take care of the basics for you and the control wiring for 6 to 10 circuit kits will easily fit through the upfitter allowance in the grommet. Don't drill unnecessary holes in the truck. There other threads in this sight that will give you all the info and part numbers to get the gm upfitter switches if you dont like aftermarket. With todays technology of led lights I cannot imagine why you need a 4 gu. Inside the cab... but if you are still using halogen I would suggest a relay or constant duty solenoid as to not have to pull that gu of wire into the cab. Good luck!