Hi,
I'm posting this to assist those attempting to wire in a third brake light on a topper since I had to tackle this project myself and couldn't find a good answer via Internet.
I tried using a "logic module" and tapping into signal lights but this doesn't work. Couldn't use trailer hook-up either because it also uses the signal light and brake light in the same bulb, different filament...meaning the third brake light would blink with either signal light on. (The tail lights is a different bulb)
Therefore I had to tap into the brake switch which is under the drivers side dash just above the brake pedal. My switch has 6-wires. Just put a volt-meter on each wire and look for a wire that is initially Zero-volts but goes to 12+ volts when the brake pedal is pushed. I had 2 choices of possible wires that went from 0 to 12 volts when the brake pedal was pushed. Then just tap into that wire and run it back to your topper. (I'm sure anyone can handle that....after knowing what power wire to use)
Hope this helps.

 

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.

 

Logic box worked OK for me. Look up the thread "2006 Topper brake light' from 11/14/2012 in General Tech & Electric and see my post (I'm too stupid to know how to print it here-sorry).