[dsraven]

how about using one you can attach with 2 sided tape to the window on the inside, up high for visibility (and that is where most people will be looking for a 3rd brake light). that way the wires can be run in the inner roof panel and be hidden for the most part and the tape will hide the reflective light from the glare off the glass.
I am using an led light from a cavalier with the trunk spoiler but mine is mounted into the roof from the outside, some sheetmetal work required.