Thanks. Yes, I read about the replacement speakers for the Miata. This is one avenue. The other avenue would be the Retrosound 4x8 dual voice speakers. They have the same shape as the ones for the dash and their wiring is stereo. This would allow fitting only one speaker in each headrest. However, they may be too powerful for a headrest application. I need to contact a guy in Florida who specializes in Miata sound systems and ask him his opinion. I took a spare headrest I have apart, and both approaches (two small 3.5" speakers or one bigger dual voice 4 x 8 speaker) would be possible, though, due to the shape of the metallic structure that needs to be modified, fitting only one bigger speaker would be easier. It's a shame that the smallest dual voice speakers I found are whether round 6.5" (too big) or 4x8 (which would fit). Smaller dual voice speakers would have been ideal.