There is a plastic retainer that is stapled to the carpet that clips over the body. I’ve removed the staples before now and then reattached the retainer using the removed staples to re-secure the carpet.
The trouble is, I think that the carpet is slightly too short so hands above the floor pan. I think some people add a bit of packing under the carpet to give it something to rest on. I can’t advise what to use to pack it I’m afraid. Maybe a few layer of sound deadening material.
I had the same issue with my 96, with the later felt carpet, and thought at first the carpet had shrunk, but I think the later cars are more affected, because Mazda used cheap felt in these later cars.
Carpet: Customer Interest
Interior - Carpet Pulls Loose at Side Trim Panel
S017/98
SHORT CARPET AT FRONT SIDE TRIM
APPLICABLE MODELS
All 1990-97 Miata
DESCRIPTION
The edge of the carpet comes loose from the bottom of the side trim panel causing a poor appearance. This is caused by the insulator pad beneath the
carpet becoming compressed over time resulting in a loss of thickness in the padding below.
Customers complaining of this concern should have their vehicle repaired using the following procedure.
Verify customer complaint.
Remove the scuff plate and the front side trim panel per the Workshop Manual section S (09).
Pull up floor carpet to expose the insulator pad as shown.
Using scissors, cut the existing pad leaving a forward leading edge of 130 mm (5.1 in.) on the driver-side and 185 mm (7.3 in.) on the
passenger-side as shown.
CAUTION:
Using a knife instead of scissors to cut pad may result in damage to the carpet.
Position the new insulator pad in place of the removed padding. Make sure that the old and new padding do not overlap.