Depending on what you consider retro, there are some simple radios that match the Miata's 25 year old age. Here are some that I've seen that are good looking and styled to match the OEM interior.
The VDO TR7311U-OR is about as simple as you get.
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The VDO CD7326U-OR which has dual knobs, a little chrome and a CD player
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The Bosch Dover USB40
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Of course the -best- would be one of the Alpine units from the era. They all look good with their green illumination that matches the Miata. The one that's long been championed as the best cd player ever, and was even re-released for it's 25th anniversary would be the 7909. Though it didn't come with USB input, and was a pullout, it can be converted to fixed install and aux input. That's what I have waiting to go in my Miata.
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