"If your stock AM/FM Miata radio does not always turn-on I've found a simple
no-cost fix that almost anyone can do.  This common problem is widely
reported on the Internet and other places.  As Miatas age the problem often
occurs in high humidity conditions and may occur when the car has not been
used for several days.  It's far more difficult to do the simple task of
removing the radio from the dash than it is to perform this "fix". 

Once the radio is removed, or at least pulled out from the dash far enough to
expose the plugs on the rear of the radio, pull out the round black plastic
plug with the finger-hold black plastic loop.  After you remove the plug look
down inside of it and you'll see four metal pins.  This plug is called SHORT
PLUG on page T-119 of the 1991 workshop manual.  

Next you'll need a piece of un-insulated solid wire to make a jumper
connection into the black socket on the back of the radio between pins #5 and
11.  Match the wire's gauge (wire diameter) to the metal pin size in the
black plastic plug you removed -- it's about 20 gauge.  Bend the jumper wire
in a "U" shape with sharp 90 degree angles.  The bottom width of the "U"
should match the spacing between socket pins #5 and 11 -- 0.22 inch.  The
sides of the "U" forming the pin connections should each be about 0.3 inch
long.  

Press the U-shape jumper wire you made all the way down into pins #5 and 11
of the radio's socket and replace the SHORT PLUG -- you've fixed the "on
switch" radio problem.  

Note that the pin numbers are labeled on the radio's socket but you may need
a magnifying glass and good light to see them.  Looking at the radio, top-up,
from the back, Pin #5 (ground) is the socket hole on the far right, center
row.  Pin #11 is on the bottom row, second from the left.  The workshop
manual says pin #11 is "NC" (no connection).  However, inside the radio it's
not.

If you need instructions on how to remove the radio, see DASHBOARD AND
CONSOLE Removal in a Miata Workshop Manual -- pages S-52 to S-54 in the 1991
manual.  One hint, don't be discouraged when removing the Center Louvers --
the pull string (rope may work better) you loop through may need to be pulled
VERY hard the first time the louvers are removed.  Don't forget, you must
know your anti-theft code before disconnecting the battery or radio.

I found through disassembly of the radio and lengthy experimentation that the
radio's turn-on problem results when a minuscule amount of moisture forms
between the bottom of the SHORT PLUG socket and the printed circuit board
that it is mounted on.  When any detectable amount of condensation forms on
the printed circuit board a conductive path is produced between pin #11 and
other nearby positive voltage sources.  The circuit associated with the
used pin #11 is a very high impedance one that if allowed to go positive
prevents the radio's turn on.  This type of printed circuit board problem
generally becomes worse over time due to the greater conductivity of water
when it is mixed with dust.  The "fix" jumper ties pin #11 to 5 (ground) and
prevents pin #11 from going positive.  I recognize that this may not be the
most professional repair of the radio's design problem, but it is one that
anyone can do for free (not counting the cost of less than an inch of wire).
 

I don't know if this "fix" solution will work with installations that have
the CD player -- I don't have the CD.  The SHORT PLUG is used to substitute
for the CD player's plug.  With the CD player plugged-in the added fix-jumper
will likely interfere with the CD player's additional plug connections.  Pin
#11 may be used (independent of what the book says) for turning the radio off
when the CD player is used.  In this case a 10K ohm resistor from pin #11 to
ground may be a solution." 

Here's another post on the same problem

From: 	David W. Thompson, 102002,3300
Date: 	Feb 2, 1996, 20:59


I've seen several discourses regarding a problem with the stock AM/FM radio in
the miata not always turning on.  One solution forwarded to correct this was to
insert a jumper OR a resistor between terminals 11 and 5 of the CD player
connector on the head unit.  The original solution had not been tested on a
setup with the "Mazda" (Panasonic) CD player.

My radio started acting up a few months ago; the CD player being in place does
not prevent this problem.  Since I and could not install the wire jumper and
still be able to plug in the CD player, I took things one step further.

1)  Remove the screw holding the top plate of the head unit (in center) and the
two on either side of the heat sink/amplifier assembly.

2)  Carefully remove the heat sink/amplifier (all of the connectors are part of
this assembly.

3)  Solder a 10K resistor between terminals 11 and 5 on the circuit board side;
steady hand is required, neatness counts.

4)  Reverse steps 1 and 2

Later.

David Thompson, 102002.3300@compuserve.com