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So, is the alarm pictured below (the cube-shaped thing with rounded corners) a standard fitting, or an aftermarket one ? If aftermarket, anyone know what brand ?

It hasn't taken kindly to being doused in boiling coolant from a burst hose and now refuses to work

Looks like an aftermarket one.
Ones installed by the factory (granted, never seen one on an MX5) tend to be a lot more hidden, for security.

I'd be talking to an auto electrician about getting it removed or replaced.

looks exactly like the factory unit fitted to my 2002 323 Astina..... You'll still need an auto electrician to get it removed.

The car is still running fine with it disabled (the car came with a little key that fits into the top of the alarm unit to turn it off). I might just leave it turned off and keep my insurance paid up

I had the same speaker unit on an aftermarket alarm. If you leave the key in it eventually bounces out and falls to the road . It can't be replaced. Store it safely or wire it in there. You can't turn the unit off without the key so if (when) it goes feral and the alarm goes off in the middle of the night you're stuffed. It has its own battery sealed inside. All you can do is wrap blankets around it to try to muffle it for 5 or 10 minutes until the battery dies. Don't ask me how I know.

Looks like the one in the old GM, which I was told (by Newells) was factory Mazda. It was giving me grief, I can't remember in what way, so I disabled it.

Interestingly, I note that in the paperwork for GM2 is a reference in a service to the removal of the factory alarm, apparently it was causing the then owner problems.

That looks like the same siren in mine Dave, the control unit is hidden elsewhere I think. I'll doublecheck again tomorrow.

The microwave/movement sensor is very sensitive, any raindrop beyond 1mL in weight impacting at 0.0000000000001 g will make the alarm chirp. Bird flys past when the car is armed - warning chirps.

Does that sound familiar to factory alarm owners?

Also, the key isn't meant to be stored in the engine bay...

If same as the Mazda alarm in mine, the sensor is a small black box up under the dash near the steering column and is easy to adjust. There's two small holes in the box (mine were covered with some tape to stop dust) that gives access to two small screw adjustments - one for bump/ movement and one for the microwave. Until I adjusted mine it drove us nuts every time we walked past it or even when another car would pull up next to it in car parks!

Yep got this one too - should be a cap on that keyhole too - works fine