Wiring 3rd brake light on truck cap

Just purchased a used Leer truck cap. It has wires going to the left rear of the cap that is for powering the 3rd brake light. Where do I access power to hook this up? I found a 2 prong adapter online and I assume its what I need to get power to it. http://www.truckoutfittersplus.com/store/topper_parts_dome_lights_brake_lights.htm#brakelight

Here is what the wire looks like.

 

Pull the drivers (or whatever side of the truck) tail light and tap into that for the brake.

When I had my first gen cap installed they tapped the parking light for the dome light power, which blew, because the dome light only had power if the parking lights were on. I wired it with a hot wire to the battery, fuse of course.

It's easy, I just did mine this afternoon on my second gen.