On your 2008 or any other '03ish and newer, you've got only a couple options. Neither are easy. Option 1: tap into your stock third brake light power wire (easiest). Or option 2: tap into your brake pedal sensor out signal at the top of your brake pedal. Looks like that worked for you.
Earlier models about '03 and older have a dedicated "JUST BRAKE" wire out back under the tailgate on the frame, even if the taillights themselves don't have a "JUST BRAKE" wire. My truck happens to be in that boat.....having 2 bulb tails yet the truck is pre-wired for 3 bulb tails.
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.