In this description I try to show how to build a Line-In to the VDO CDR 2005 radio from Opel (GM). For this you'll need to disconnect the radio out of your car, open it an do some soldering works. As far as I know if you do this you will not have any guarantee after works.
1. First of all you have to get the Radio out of the car and open the lower side (bootom):

2. In the next Picture you can see where to solder normal flexible wire for left and right channel an ground:

3. The other ends of the wire you connect to a normal "3,5:

4. With this method you're using the connectors of the CD-Part of the radio to put in a external Audio-Signal. This means the Radio has to run in CD-Mode so that you can hear your signal. The only way to get this radio into CD-mode is to put in a compact disk and play it. For that I generated a CD with only one track of 74 minutes with only silence on it. To get your mp3-player running you have to put in the new "Silence-CD", plug your player to the "3,5 mm. and press the play button. Every 74 Minutes there will be a small break when the CD begins to play from the beginning. You can use you remote on the steering wheel if you have one to control the radio and it will interrupt if there is traffic information, just as if you played a normal CD.