[Garffey]

I fitted one similar to the one in the photo to the rear of my Vitara inside the rear window and it worked ok, unless the rear window was steamed up....Then on my next Vitara mounted it just above the number plate but water got in, I then got a 'truck' one and its been fantastic, but would sadly be rather big on the back of an MX!

 

[Garffey]

On the Mk3 there's a black rectangle on the underside of the boot lid between the number plate lights, does anyone know what it is or if it's ok to remove it so I can try to mount something there?

I'd be thinking along the lines of possibly using the blanking plate as a place to mount one of the angle type cameras....you can then just clip it back in place?
Like this one..Connects2 CAM-24 Universal Surface Mounted Fixed Angle Reverse Camera

 

[Garffey]

Auto link had them in stock...although they may not have updated for new stock.
Boot lid hole cover, centre, MX-5 mk3, NF4767H41, USED
Looking at the picture you'll have to modify the back of the blanking plate to accept a nut and washer... I'd reinforce it with a piece of steel/ally just for a bit more strength.