Hey all- My MX5 was killed by an inconsiderate Chrysler 300 last year. I was about to replace the drivers door mirror at the time (glass fell out after losing the black trim strip). So, I have the glass left. Item is over $80 to get the whole housing from Mazda (they do not sell just the glass mirror). Quick fix if yours is cracked and the housing is OK.
This link below should take you there or go to ebay and search for seller:
66CL77 or item number 7931950993 or MIATA MIRROR
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Tim