Topper Center Break Light

by tapping in to the brake circuit before it goes to the turn signal switch; which means at the switch it self. Or if it has a third light on the cab roof, tapping into its circuit. You cant do it at the tail lights, or from the trailer harness. Whitelightning

depending on the year of the truck there should be a trailer brake light wire running to the rear somewhere

You don't mention the year of your truck. On my 04 there is a wire next to the 7 pin trailer plug just for the center mount brake light. If you don't have this you need to tap into the trucks center light. Find out which pillar the wire runs up. Remove the cover to that pillar. Tap into the wire and pass that wire out of a cab vent or drill and grommet a hole.

Thanks for all your help ( the year of the truck is 1999 )