Hi Andre,
I´ll try to help you. First of all , you have to remove the dashboard part covering the audio head unit. That´s quite easy, you just have to pull and easily it will be removed. Then you have to unscrew the audio head unit, the airconditioning control and the doors´ lock unit. This means 10 screws.
Now you have access to the rear part of the head unit where you can see two connectors.
The one on the left has four wires (black , red, blue and white). That is our connector. We have to connect the audio device output to the black=ground, blue=left and red=right. I did it by cutting the wires and putting two connectors but you also can weld them.
I hope this to be useful.
jcarlos