Step 1: First find a schematic for your paticular radio.
The first thing is to remove your radio from the car. In my case, there are two holes on each side of the radio. You have to put a stiff wire, like a coat hanger in the two holes on one side, push hard to release. Then release the other side. There is no good way to get a hold of the radio. I just stuck my fingers in the cassette opening and pulled.
This is the hardest part of the whole project.
Then I made a simple drawing of the wires on the car side of the plug going into the radio. (Sorry for the fuzzy pics I didn't have my good camera.)