tms,
Welcome!
I've thought about a 3rd brake light too... I guess the height of the regular brake lights qualify as being high enough to do the job of third brake light also.
If you have the trailer hitch option and wiring, you could easily splice a light to the brake light feed there.
If not, it's just a bit more complicated because you can't splice to the exiting brake light with another light because it would bee too big a load on the brake light circuit... and probably get the Canbus system all upset at you. In that case (no trailer wiring), it would be really easy to add a relay to the brake light wire in the back corner of the van, then use the relay to feed 12V to the 3rd brake light.
I think the camera housing would be too high to be effective. I was thinking about putting a light on the passenger side rear door in the area symmetrically across from the license plate. The wire to feed it could be added to the harness that is in the rear door. Or, if you have windows in the back, mount the 3rd light at the bottom of the window on the inside, like they do in cars.
Haven't moved forward on that project... yet!