You need a test light, pocket knife, spare wire (same gauge on camper/PSD), electric tape. Maybe some electrical connectors and electrical pliers/crimper if you want to be fancy with it. With the tester, crawl under PU and locate either the wires leading to the PU brake lights or the trailer wiring before the female plug system. Have someone step on brakes and then you test for the 'hot' wire. Splice this wire into your shell red wire. Take the white one and ground to PU frame. The white ground wire is the weak link. Make sure to put in good location somewhere for corrosion protection. Tape up any splices and connections real good with tape.
This is only valid for normal BRAKE light functions. Running lights, turn signals, etc on a shell needs to be routed to corresponding wires on PU. If you also have these to do, buy yourself a 4/5 plug in type system just like a small pull camper or trailer. Everything from shell to trailers will use one spot this way. Will save you frustration in future when a light(s) doesn't work properly or the shell comes off.
I originally wired mine that way and my third brake lamp on the shell would blink with the left turn signal.
I had to drop a wire from the cab's third brake light and run it down the frame and up into the shell to connect to the shell's brake light. Now it works as it should.