Sony multi-disk changer.
It mounts on a solid red oak base that I spent way too much time grinding to fit exactly into the spare tire. Keeps the changer out of valuable trunk space and in hard-to-use trunk space. Someday may mount spare below trunk floor. The car does have a factory in-dash cd unit, which is a piece of junk. I lived with it for about a year, and one fine day after it skipped about 100 times in a mile, I couldn't take anymore. The Sony only skips when the tail bites after rather severe induced oversteer, and not over bumps. Actually, since I've had it mounted, it only skipped once.
Rigged the power antenna to a switch in the armrest so it is only up when not listening to the CD player, which feeds into the FM antenna. For those who obsess over sound quality, FM output is a standard add-on CD player interface. In a car with this much extraneous noise, it just doesn't matter. Yes, the player has line level outputs which I would use in a more serious set-up.
The aft gadget.
Also features a set of CRI horns. The horns are one of the first things I got for the car, and though they are a bit more expensive than generic air horn kits, the few pieces that make them Miata-specific, plus the great instructions, are well worth the several hours that a Miata owner will save over adapting a generic kit to their car. They're quite loud, and people definitely pay attention. If they don't see the stripes or driving lights, when they hear that horn they must check that there isn't some large German sedan bearing down on them...