Nice setupI've been meaning to do this for awhile...
This is what I used on my truck (99 K2500 Sub) On the left kick panel distribution block is a spot marked "NAT", just above the relays. The right terminal is the cold side of the brake and the left terminal is the parking light circuit. USE YOUR TESTER TO BE SURE.
Put your test light on the right terminal and push the brake pedal - if it lights, you're in business!