Hello Mdlestmsg, first off let say I am a little bit OCD when it comes to wiring, I take my wiring connectors apart and solder the new wire onto the crimped end then reinstall the pin into the connector.
Ok, you are saying the shell door has a power lock. I would tap into the tailgate lock wire, and round the wire along the harness that runs up to the left tail light and into the bed on front side of the stake pocket.
Did you find the factory supplied wire for the 3rd brake light or did you tap off one of the tail lights ?
If you have factory cargo lights next to the tail gate, I would tap into the left one for the interior lights (if the shell is equipped).
Last suggestion is use a flat 4 pin trailer plug between the truck and shell. That way if you need to remove the shell just unplug and remove the shell.
I will post some pics of my shell and wiring job.
