1. Support driver's side of Miata securely on two jackstands. Lay under car. Working above the gearbox from the left side, unplug two reverse light switch wires. Exercise caution when reaching around the hot exhaust pipe.
2. Remove switch with a 24 mm open end wrench. It should not be incredibly tight. Large pliers will work in a pinch (minor scoring of switch body will result). Oil will not spill out; this hole is above the oil level.
3. With a short sturdy prying tool, reach into reverse switch hole and pry reverse rod end toward back of car. A stubby 3" straight blade screwdriver is ideal. Take care to avoid damage to the threads in aluminum gearbox case.
4. Rod should click into neutral position. Metallic clicking sound might be 5-R rod dropping back into reverse position. To confirm successful repair, verify correct operation of shift lever. Repeat if necessary.
5. Replace switch to keep oil in and dirt out. Reuse copper packing if it is in good condition. If this is a field repair, use packing regardless of its condition and get a new one later. Plug in wires; they have no polarity.
6. This field repair will completely and absolutely return your Miata to the precise operating condition before the malfunction. The further remedial repairs are not required.
Pulled from M.net