I, too, have thought about putting a backup camera where the third brake light is located. I was reminded of that idea recently when I looked into the third brake light (details here:
http://b190.com/viewtopic.php?f=2&t=1475&p=5843#p5843).
I'm currently imagining mounting a camera directly inside the stock third brake light fixture; probably by drilling a hole for the camera in the red lens (some cameras are designed to mount in a small round hole), then routing the wires through another hole in the brake light body. I might at the same time convert the fixture to LED, being careful to leave room for the camera.
The camera would likely have to be angled relatively downward, in order to include in its field of view the area directly behind the van. In this way a camera with a fairly wide viewing angle might include both the area near the van and a view to the horizon.
(As an aside: considering that the area of interest is a sort of vertical strip behind the vehicle, from the rear bumper up to the horizon, the standard wide video format seems a poor fit for our needs. After all, my side mirrors show the view to the left and right of the vehicle. With this in mind, could we turn the camera sideways? Of course this would only work if the in-cab view screen could also be turned sideways (i.e., to "portrait mode"), which won't be possible on many types of screens.)
I think cable routing from the location of the third brake light to the vehicle cab should be fairly easy; as I recall, there's a sort of trough formed by the original van body that runs most of the way around the vehicle. The third brake light seems to be directly in that trough. I previously ran a pull string most of that distance for possible future use (such as this).
I welcome feedback about all this. How can we make it better?