I found a thread that outlined installing a flasher on the third brake light. With the car being so little, I thought that was a great idea so others can see me better, especially when stopping. It is a low cost safety feature that works.
One of the links is http://www.miata.net/garage/brakeflash.html . It outlines using a flasher that can be purchased at any auto parts store. I followed the instructions and was successful. I don’t have anything to add to the instructions. I just liked having the extra warning for the vehicle following me.
This is how I connected mine.
I could have used a vampire tap for the Black ground wire, but this was the first one I installed and I used solder to the terminal. The Green wire is intercepted to make the light blink. The flasher is just tucked into the area behind the light out of sight.
On one of my cars the light did not blink when tested. All I had to do was check the connections to the flasher and move swap the two leads from the Green wire.
Now all of my Miatas have flashing third brake lights.