For 2004-2006 the Mazda3 hatch Center High Mount Stop Lamp had a single incandescent “921" bulb behind a red lens. 2007+ repositioned the lamp to the spoiler, and was a clear lens with red LEDs.
Having a surplus of red LEDs salvaged from the running lamps of old warranty scrap RX-8 tail lights from back in my Mazda Parts days, I decided to upgrade my CHMSL, without the cost of swapping the whole spoiler which is one way of doing things.... the other would be to buy a “921" bulb that was LED, but I already had some. And I’m cheap.
I wired and zip-tied the LED boards together into a string 6 wide and 2 high, which fit the dimensions of the brake light cavity perfectly. I crimped on a couple terminals to push into the bulb socket, and BAM. Brighter. Faster. Redder.