I've added a glovebox light to both my Miatas. It's not hard to do the way I did it.
I couldn't find a momentary switch that would install easily in a conventional way that would look good to automatically operate the light by opening/closing the glovebox. So, I elected to install the switch hidden from view behind the glovebox.
Under the dash, below the glovebox is a metal strap that already has holes that I used for inserting bolts to add a metal bracket to support a switch.
I got a heavy strap of metal and about 1 inch from the end I bent it at 90 degrees for mounting to the car's strap below the glovebox. On the long end (about 3 inches) of the added strap I drilled a hole for mounting the monetary button switch horizontally.
The switch's plunger button strikes the outside back of the glovebox when the glovebox is closed and turns the lamp off.
I picked up the 12V power from the nearby dash light (depending on the year of your Miata you may not have this nearby dash light). On one Miata I used a small lamp that has a spring metal clip for push-on mounting the lamp to a plastic flange up under the dash just above where the glovebox door first opens. On my other Miata I mounted the lamp to the side of my electric glovebox lock.
See rough drawing below:
xxxxxxxxxxxxx x install x x lamp in x x this area x xxxxxxxxxxxxx |<--back side of glovebox / g | front side of glovebox-->/ l | / o | glovebox shown closed / v plunger | / e switch | | / b ____| | / o | |===|| / x |____ |===|| / | | / d | | / o added | | / o bracket->| |________/ r |========|| |=========| <---- car's existing metal bracket
<--- direction to front of car
DRAWING VIEW IS FROM SIDE OF CAR