mx5cartalk.com

i assume your talking about the key for the battery backup siren

i dont think they are all the same BUT i could be wrong

optional factory most of the times are aftermarket but branded Mazda

you could replace the entire siren which will inc a key

Could pull the alarm apart and get a locksmith to try and make a key or lock for it.
It's stuffed without a key so you have little to loose

Be aware the car may become un-drivable if you randomly rip it out or break it.