Somebody nicked the old Radio before I bought it. So I bought a new one which plays MP3s.
I got a small bit of strip board (Vero board) and labeled 18 strips with a solder blob at each end. If you can solder then Terminal Strip might do.
I then found 0 volt and +12 volt source by putting a plug into the cigarette lighter socket.
All the wires hanging around on the old connectors to the radio I chopped off carefully one by one and very carefully in no particular order soldered the onto one side of the strip board.
With the ignition key to off, I used a cheap multimeter set to 200 Ohms to connect each wire to all the others through the multimeter. the reading on the multimeter was 4 Ohms for my car. (8 of them should be Speaker wires. My 4 track had 4 ohms speakers but only 2 of them)
GET A NOTEBOOK ! and against 1 to 18 write down which pairs showed 4 Ohms. Enter details of EVERY wire identified.
Then attach the Multimeter to +12v on the cigarette lighted plugs lead , switch the Ignition lead to ACC and measure the voltage on each non speaker wire.
One or two should read 12volts.....these are you Ground , o volt wires.
Now attach one of the multimeter lead to the 0v line on the lighter socket. With the other lead probe all the unkown wires. Likely there will be only two. This is your main power to the Radio or the power line which holds up the memory within the Radio of your favourite radio stations.
Now turn the Ignition switch to OFF and do it again.
This is the real Memory back up wire ! (Missing on mine so I ran a wire through a fuse to the Vero board from + on the battery.
The other +12v wire is the main Radio wire.
There are a few other wires that maybe of interest such as operating a Power Aerial etc. But if you connect you radio wires (or matching lead and plug for your radio) you maybe able to suss out how the rest of the wires work.
Hopefully these simple instructions can be polished to cater for a variety of Radios.
PS to test which speaker is which just tap a 3 volts dc source across its wires. It will click ! Two AA batteries are ideal.