The Miata fenders have a habit of trapping debris which can lead to rust. Gary Samad, Douglas Dinsmoor, and Jim Hayter contributed to this bit of wisdom on cleaning out the crud.
  1. Look for two bolts at the bottom edge of the fender just in front of the door and just under the car.
  2. Using a 10mm (I think) socket wrench, remove them. You may find that the socket doesn't fit over the bolt at first because of the undercoating. Just force it on.
  3. The base of the fender now hangs free. Work a toothbrush between the fender and the body and brush it back and forth. You should see lots of dirt hitting the ground. It is important to face the toothbrush in both directions but especially toward the fender, since this lip catches most of the dirt.

This exercise should be done routinely, but there's an easier way:

Take out the 2 nylon screws and the bolt that attach the fender near the wheel well, and the whole fender flops free. You can stick your hand in there, or even wash it out with a hose. By taking it apart more, you make sure you do a thourough job, and get all of the big pieces.

Most of the detritus that ends up hear comes from the drains under the hood, in the corners near the windsheild. I've been thinking for a while of fashioning a couple of pieces of screen to fit in the drain holes to keep out the pine needles and leaves that end up under the hood.