Mazda Miata is a very small car and sometimes drivers following in bigger cars might not notice right away if you are starting to brake. To alert their attention, a flashing 3rd brake seems like a good solution.
There are many modules available on the market; for example, the GS-100A, sold under multiple brands and are very inexpensive. Installation of such devices is simple, but requires you to cut the wire from the 3rd brake and attach it to the hanging wires on the flasher module.
I was able to source the same type of connectors as Mazda used for the 3rd brake connection, and made a simple kit, which simplifies installation to a plug-n-play type.
Installation do not require to remove any panels.
This module will produce a strobe effect for a couple of seconds, flash a few times, then stay solid afterwards. For sure this will be very noticeable to drivers behind, but I think it will also be very annoying during stop-n-go traffic.
I designed my own version with integrated accelerometer. It will flash only during hard braking, when deceleration exceeds some threshold.
Picture below shows first prototype.
To test this version to find an optimal threshold, I installed this device inside the cabin and connected a LED strip for visual control. Power comes from the brake light wire, so I have a perfect simulation.
Legality of such devices is not very clear, and different for each state. For California it is legal to produce 4 flashes during the first 4 seconds of brake application. My current implementation will blink 4 times, with 400ms period for each blink, when the accelerometer detects force -0.3G in forward direction.
One more discovery was that the Mazda MX-5 RF model uses a different type of connectors for the 3rd brake light. Fortunately, I was able to source these connectors as well.
Both versions will be available through my store
Discussion on Miata.net forum.