Anyone got any advice on the best way to hook up the 3rd brake light?

 

Pull your taillight out of it's mount and determine which wire is for the brakes. Tap into that wire and make sure you leave some slack. You will also want to get power for the interior light inside the topper and for that I tapped into the wire on my towing harness. You just need one of those test lights that actually pierce into the wire. That's what I used anyways....

 

I have an 01.5 2500 so it may be different. In a wiring harness driver side towards rear of truck underneath the bed There was a wire hanging out of the loom (maybe 2 inches long) with a small tag on it labeled "chmsl" (center high mount stop light). I think it was just before or just past the axle. I didnt use it for a topper but for the brake lights on my headache rack since it covered the original highmount light. Doesnt solve the interior topper light if it has one but it gets you half way there if it is there on yours.

 

If yer going to pierce wires under the truck with your test light, be sure to dress the wounds with a drop of liquid electrical tape, or a couple of years down the line you'll have all kinds of mystery problems where the wire corroded.

 

BarryG: Thanks for the info on the CHMSL. The wire was there just like you said, right next to the spare tire. White with a little black tip covering the end. Stripped it back, tapped in right there and third brake light on the topper works fine. Nice!