Miata Mailing List: April 1996, Message #56
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   From: 
Subject: Headrest speaker wires
   Date: Mon, 1 Apr 1996 17:15:19 -0500

    The extra two wires on the connectors are probably used to ensure the 
headrest speakers are wired in series (signal from amp in positive connection 
of speaker 1[passenger right]; signal out negative connection of speaker 1 and 
then in positive connection of speaker 2[driver right]; signal out negative 
connection of speaker 2 and back to amplifier).  Repeat for passenger 
and driver left speakers.  Headrest speakers are usually 4 ohms (ok load for  
amplifier).  Wired in series, resistance adds (4 + 4 = 8 ohms) and is still no 
problem for the amplifier.  If the speakers are wired in parallel, the 
resulting load is 2 ohms which is too low for most inexpensive amps.  
High-quality amps will handle 2 ohm loads but the stock amplifier safety 
features will probably shut it down because of an over-current condition 
caused by too little resistance.
    The above is greatly simplified and subject to review of the schematics of 
the car.

The (Yes, I did wire my own speakers into the headrests in series and I 
was not aware of nor found the factory connectors) BirdMeister