Joe: I found suitable third brake lights in the junkyard (at no cost,
thrown in with other junk I was buying) for both of my vanagons.  In both
cases I ran a ground and hot wire (tied in to the brake light bulb wire)
up from my left taillight, behind the cabinet and across the a/c shelf to
the light.  I was able to find units that had an angled rubber skirt
(designed to minimize the effect of sunlight shining on the lens) which
matched the vanagon rear window.
Didn't have to worry about tinting but the fellow that mentioned cutting
the film away in the shape of your light has the right idea.  Also,
remember to mount the unit far enough away from the window to allow your
curtains to close.
See some pics out at
www.weidefamily.net/vanagon/
Go to Electrical / 3rd Brake Light

It's an easy job that makes a big difference.

cheers
Brent