A few ideas:
The maintainer MUST have a blocking diode to prevent the solar panel from draining the battery when its dark. I think most if not all maintainers are equipped with diodes. Check the packaging for a statement about it.
On some cars the "cigarette lighter" power socket is on line all the time, on some its controlled by the ignition switch. I don't know what various models of Miata provide. It can be tested easily by plugging any device into the power socket (cell phone) and seeing if it functions when the car is shut off. If the power socket is not live when the car is off, the battery maintainer will have to be attached to the battery with alligator clips or a more permanent battery connection.
On my 2017 mx-5, I just use an AC powered motorcycle maintainer (1.5 amp/12 volt max) with alligator clips - haven't tried the power socket because it is not in a convenient spot.
Some "smart" chargers wont charge a very discharged battery. Give it a good charge with an AC powered battery charger first, then try the solar panel.
Be sure your battery is not defective. If it won't hold a charge when completely disconnected for a week or two it needs a replacement.
Be sure that the maintainer is working - the "charging light" should come on any time it is exposed to sunlight.
I use one on my infrequently used fishing boat and ATV. My 12 volt batteries are always at full charge - it should work fine if used correctly.
NC Miata battery switch
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