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I presume your radio is similar to the one I had in my previous '90 NA. Let's say you've unplugged the connection from the radio and you are looking at the END of the mating connector bearing in mind some of the Xs in the figure below are empty terminals.

ABC XX XX ..........
DEF XX XX ..........

A should be a solid +12v red wire which powers the radio.
B should be a solid +12v yellow wire which retains the clock and station setting after you've turned off the radio.

Hook the negative wire from the battery to anywhere of the metal chasis.

You can either use a 12v battery or a 12v power adaptor. I'm using an 16v adaptor stepped down to about 13v to run an old NA radio/cassette player in my garage.

What I described above is the view of the disconnected male plug end which I presume you still have. Otherwise it will be fiddly to hook up the power source and the speakers to the female receptor on the radio itself.

OK looking at your picture, we can see two female receptors. The top receptor is for the headrest speakers. To test the radio, you want to hook up to the bottom female receptor. Let me redraw the diagram to mirror what I described in my first post above as follow:

HG -- CBA
ON ML FED

A - Accessories +12v(activated by ignition key)
B - Constant +12v from car battery to keep the clock and station settings
C - Illumination activated in sync with parking/headlights
H - Right speaker +
O - Right speaker -
G - Left speaker +
N - Left speaker -
M&L - unused


To test your radio, you only need to hook up a +12v source to A, -12v to the metal chasis and a pair of speakers to the other connections as described above. Use an in-line fuse just in case you short your wires.

For the top female receptor which powers the headrest speakers:

E - - BA
H GF DC

A - Right headrest speaker +
C - Right headrest speaker -
F - Left headrest +
G - Left headrest -