Seon,
My father and I currently have our 58 torn apart and if your vent is like ours their is what looks to be a metal plate screwed and rivited to the back. I haven't done it, but it looks like if you were to remove this plate the slots would be open, allowing you to replace it with a translucent red lens. Then with the addition of some super bright leds in the cavity behind the vent you could have your 3rd brake light while still retaining that cool looking vent.
just an idea, Neil