We found the brake light switch tap on the long connector attached to the steering column. It was the white wire, second or third from the end (from the back of the truck) The picture shows looking up from the floorboard.
Get a test light and test all the un-used pins on this plug, looking for the one that lights up when you press the brake pedal. If you can not find any that light, you will forced to find the brake light switch wire under the dash. The brake light switch is very hard to reach, but with a probe tester, look for a wire under the dash that lights with the brake lights. I you are probabally looking for a small gauge white wire (seems most GM trucks of this era used white). Tap into the wire for the brake light feed (red wire on most controllers)
For power, we ran a wire through the firewall, and tapped onto the fuse panel under the hood.
We grounded the controller under the dash to a ground screw. Easy.
The blue (brake wire to plug) was simply passed through a hole in the firewall and ran along the frame to the back plug on the truck. (making sure to use lots of tie wraps to keep the wire awy from hot and moving parts)
We used a 30 amp fuse to power the brake controller, AND supply power to the 12 Volt power terminal on the trailer plug. This truck did not have wire ran to the back, so we used our 2 conductor brake wireand ran it ourself. Drilled a small 3/8" hole in a rubber plug in the fire wall to run the wire to the brake controller under the dash.
We have found on some trucks, the wire to the backs of the truck is way down under the brake master cylinder: