Where is the drain plug on a 1999 miata?

The Oil Drain Plus is on the back of the oil pan. If you lift the car, look underneath, just behind the front passenger (assuming Left-Hand drive!) wheel. You'll see the oil pan and the drain plug.

If you're looking for the Radiator Drain Plug, lift the car, and look just underneath behind the bumper. There is an oval hole about 2" long on the large plastic splash pan. You can use a Phillips head screwdriver to remove the radiator plug.