Brake Controller

I've had a controller on a 08 Titan, an 10 Avalanche and a 97 F150 and ALL have had a plug in harness. The harness was always 20.00. Still using the same Techuseh controller. Still works like a champ

There's no way I'd tap a wire on a 2014. On newer vehicles you may screw up the computer. I haven't tapped the brake switch wire on a vehicle in years. Even for my '02 Chevy I just got the plug in harness. I think it was around $20 or so. You should be able to get a harness that plugs in under the dash and has all the wires you need for the control.

Use a circuit tester and see which one goes hot when the brake is depressed.

On many vehicles that are using digital technology, tapping into a wire is now a no-no. Have to use a proper connector and interface. Can't do it on my Ford. Heck, even my 2015 Harley uses digital signaling, have to buy special switches with the proper wiring harness to attach anything to a wire. Don't know about your Chevy.

E Trailer usually has some pretty good info on things like this.
You may even be able to get a plug and play cord for a small price.