How to add an aux cord to a factory fm car radio?

I have a 2003 ford ranger that has an old factory radio that I d like to add an aux cord to. The radio only has fm and am stations no cd or cassette player. I do not want to go the route of a FM modulator. I have completely took the radio apart and my initial plan was to solder the left and right wires of the aux cord onto the fm or am outputs. My problem is there's no writing or indicators of what pin is what, it is a very old cheap board. What would be the best way to put in a aux cord to this radio?

What you're doing is just nuts. You have no schematic for the stereo so you were shooting in the dark- that can't work. It would have been so much easier to install an FM modulator and you would be happy with it. You just don't want to do things the way they work.

It's 2017. What are you doing driving an old 2003 Ford Ranger?

It's time to toss that vehicle into the landfill and get a badass 2017 Ford Mustang. If you like Ford vehicles then I think you will like the 2017 Ford Mustang. You can neither get the manual transmission or the automatic transmission.