Principles of Operation
WARNING: This page is about a different car, the 2012 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition is in run or accessory, the upfitter switch assembly receives voltage from the Body Control Module (BCM). The upfitter switches control the 4 upfitter relays in the upfitter relay box. When the relay is energized, voltage is supplied to the customer installed accessories.