If you have the factory stereo in your car and you want to put an amplifier, then you will find difficult, as the most car stereos don't have the connectors. However, you can put those connectors back and even at no extra cost.
You can do this in case you can not buy another car radio for a number of reasons, such as the fact that it does not fit in the desing of the console or that you do not want to give extra money.
For example, in my Colt CZ3, I wanted to connect Line Out to the amplifier but the radio did not have the proper connections and I was forced to reduce the level of the stereo outputs before sending it to the amplifier.
In the European forum the users reported that there was nothing to do with the connectors so I had to look at its "internal".
What you need to do is open the stereo and see its board. Here somewhere there will be the "central" integrated chip. Usually cheap radios have a simple one, like the PT2313L.
What you need to do is to google the chip's code and find its datasheet.
There you will find each pin. Let's say you have the above chip, then to put Line In and connect eg your iPod or other mp3 player, then you need to connect two female RCA jacks, where (-) of them will go to ground and the left (+) should be connected to the pin 16 and the right (+) to the pin 6.
For example, my car radio had Sanyo's LC75410E and the pin I needed was 30 + 31 for pre-out connections.
I have no photos, but the solution is simple and pretty much the same in all car stereos.