After upgrading my seats to early Miata Seats (Click Here), I finally decided to open up the headrests and install speakers. The nice thing that Mazda did was they carved out the foam and had it all ready for the speakers to go in, highly recommended for anyone who's swapped seats. I ended up using some leftover speakers (~2.5 inches) that I was getting ready to donate. After a bit of fishing wire through the seats and connecting them to the speaker feed in the driver's rear corner of the trunk, I was quite pleased with the results. As I'm still using the Datsun OEM radio, which is mono, all speakers sound the same, but when wiring I chose to make the connections and wiring an easy switch to stereo if I ever upgrade the head unit.
For anyone looking to but a complete retrofit kit, check this website out -- http://www.3rdstrategy.com/
Here's a great write up from a Triuph guy -- http://tr6.danielsonfamily.org/MiataSpeakers.htm
