Ford has four wires already running through the firewall to use with your upfitter switches. They come out of the wiring bundle near the brake master cylinder in the engine compartment and are blunt-ended under the dash (behind the fuse panel) and labled as customer access wiring. When you're hooking something up outside the cab you can tie the under-dash wires to any of the 4 upfitter wires coming off the back of the fuse block to get them to the outside without having to make new holes or figure out where to run the wires. Works really slick. I've used mine to wire aux driving lights, aux back-up lights and the high idle mod. Switch one and two are rated for 30 amps and three and four are 10 amps. The wires are sized accordingly. Hope this helps.
From the SDD WIKI:
http://www.superdutydiesel.com/wiki/images/chart1.jpg
All this info and add'l pics located in the WIKI. Follow this thread:
TikiWiki : Upfitter Switch Kit
There is no need to drill through the firewall to wire accessories to the outside. Ford has provided four 14-gauge wires that lead from the passenger compartment to the outside of the cab. They are located in a bundle behind the fuse panel above the parking brake. There will be a tape marked “CUSTOMER ACCESS” around this wire bundle. There will be 11 wires total in the harness. The other wires are used to control such options as power-take-off (PTO) and provide additional feedback instrumentation. See the illustration below:
These pass-through wires terminate in the engine compartment in front of the cowl on the driver’s side, behind the brake master cylinder. Any wiring attached to these blunt cut wires should be made with moisture-resistant connections, and the wiring should be of an heat resistant type with polypropylene insulation per Ford recommendation. Ensure that the wiring is kept well away from the heat sources of the engine and turbocharger.
They are identified as follows:
http://www.superdutydiesel.com/wiki/images/chart2.jpg