It should not matter if you mess up the wiring of the two +12 V wires, if those are switched around the HW kit just would not work correctly, it should not blow it.
My friend have a Mercedes wagon,,,,, funny enough with the fuse box in the back, so putting a camera in his car he did not want to use the dash socket, so we got hold of a 4 port USB charger to use that hardwired to a ACC fuse.
The problem however was with a multi meter i could not find a damn ACC fuse in the car, they was all on all the time. But as it turned out it was just the smarts in the wire harness, so after we let the car sit for a while i was able to find a fuse that was now off.
But your 94 Miata should be even less smart than my little Suzuki car from 2012, and the smartest thing in that is the airbags and the ABS / ESP system, no problem finding always on and ACC fuses there, the only problem is reaching them in the little POS car ( i doubt a American would even see his worst enemy drive a car like mine )
My fuse box are in the drive foot well up under the dash, so you have to be a contortionist to reach them,,,,,, and at 56 years of age and a body ravaged of working too hard too much, well i am more like a 2 X 6 plank now.
If your HW kit are indeed bad, just hit your seller up and a new one should son be on the way to you.
PS i assume you use the street guardian hard wire kit,,,,,,, you need that as the time of generic hard wire kits are over.