I just finished finding this on my 2013. We needed to use that wire for a different purpose, so I called FW and they told me to look for a wire cluster hanging near the brake pedal. We had to poke around a bit because there are lots and lots of wire clusters there, but sure enough we did find a cluster with about 8 wires in it that went to a "dead-end" plug. Had to remove the end-cap of that plug and then use a tester to determine which wire was which. I was merely a helper in this process--using some testing equipment that my mechanic had. He plugged in a pigtail on my RV's 7-pin connector, twisted two wires together, then we tested each of those wires by the break pedal by touching the end with the tester and simultaneously hitting the brake pedal. Hitting the brake pedal completed the circuit, enabling us to locate the correct wire.
I hope you don't ask for more details, because I really wasn't 100% clear on what the heck we were doing! But I was skinny enough to easily get myself under that steering wheel, whereas my mechanic... well... he told me he was on a "tea" diet, but he was referring to a different T: Tacos, tortillas, tostitos..
2013 Fleetwood Discovery 40G, Cummins ISL,Freightliner Chassis, Allison 3000.
2017 Ford F150 Lariat toad w/
Blue Ox Aventa tow bar and M&G Braking System