There is a video on YouTube called "DanDak MX-5 NA 3rd brake light LED conversion kit" [:

Stock bulb:

• Original Mazda’s solution to use incandescent light bulb is out of date
• Most (90% and more) of regular bulbs power dissipates as heat
• Short lifespan – up to 1000 hours when running constantly without switching

LED conversion module:

• Uses 7W of power instead of 18W standard
• Brighter
• Spreads light better – LEDs are spread across the whole surface of the light
• Plugs right into the same socket with included adapter

This DIY kit includes:

• LED conversion module – has to be glued into a housing, plugs right into your old bulb socket, no soldering needed.
• Tube of glue to hold the board in place

You can order this kit in 1 of 3 different versions, please mention it while purchasing. 

• Simple – all LEDs on simultaneously
• Classy – sequential which starts from the centre, keeps all LEDs lit afterwards
• Race – flashing to bring maximum attention (could flash endlessly or your chosen amount of times then - solid)

Contact me if you have any questions, Dan