The price of chrome or billet 3rd brake-lights gives me a headache so I decided to fabricate one of my own. I scrounged all the pieces for this one from my junk bins but my guess is it would cost you $3 or $4 if you had to go out and buy everything.
This piece of junk provided the basic idea for my light. It's from a TV antennae my brother-in-law tossed out years ago. The aluminum center section of this piece will make a nice "housing" for the light.
The major challenge on this little project was how to make a lens that would fit and stay secured inside of the tubular aluminum housing. To do this I used some broken and discarded plastic dropped ceiling lighting tile. The material looks like this.
To curve the lens to fit inside the tubing I used my heavy duty heat gun. I'm not certain, but I think a decent hair dryer would also do the trick...just take a little longer. I heated the plastic and bent it around a piece of pipe that was a slightly smaller diameter than the inside of the aluminum tubing.
You'll want to bend the lens material so it is just slightly larger than the aluminum tubing. That way, you can compress it enough to slide it inside the housing and when you loosen your grip it will hold itself in place by friction. Here's a shot of the lens being test fitted. Once I had the lens to the right shape I painted it using an aerosol can of Candy Apple Red paint (the type commonly used for models and sold in any hobby store).
You can "let the light out" in a number of different ways. I chose to cut three horizontal slits in the tubing using my 4 1/2 grinder. You could also drill holes or cut slits in the vertical direction rather than horizontal as I did. Note how I secured the tubing while it was being cut by clamping wood stops on every side.
I then sanded down all the rough edges on the slits, buffed up the housing, and inserted the painted lens. Presto...third brake light.
The bulbs for the unit are inserted from each end and are friction fit by wrapping foam around the base of the bulb. In place of socket connections, I soldered wires directly to the plus terminal on the bottom of the bulb and to the negative surround at the base of the bulb. The end caps for the unit will be fabricated from oak to match the oak trim which goes around the rear window. These pieces can not be made until the car comes back from the upholsterer and I know the exact size to make them. So I'll update this thread when the unit is finalized and installed.