You asked for feedback, I think it's a waste of time.
The third brakelight is there as a backup to your primary brakelights, to help save us from those that don't correct deficiencies on their vehicles.
If you were going to do it right, you'd also add another brake light switch, and run an entirely separate circuit.
I only say it's a waste of time if you are talking about doing it for the safety factor, if you like the way they look, then hey, that's personal preference. Just don't be "sold" by the fact that every new vehicle has one. If it weren't mandated by law (Libby Dole IIRC) no vehicle would have one.
Our tops are supposed to have a steel reinforcement in that back area, I'd be worried about compromising it's integrity at this point, not to mention how far the tailgate glass goes up in it.
It doesn't look like your truck has the factory spoiler? That might be another option, use those hold downs for a light up on top.
Funny thing, I've actually got a retrofit GM third brake light (actual GM piece) but I'm not sure what it fits yet. Came in a lot of stuff I bought on ebay. Looks to me like it's for the 78-88 A/G bodies, which got the third light in '86 I want to say.