there is another way to put an aux line-in for the CDR 2005 and probably other models. here it is:
1.
http://www.css-haupt.de/vectra/english.htmleasier, works through the CD-player. it simulates that the audio signal comes from the cd. you must put a data cd or an audio cd full with one 74min track of silence. I made mine like this, put a a data cd and it works perfectly. and I ran an audio cable to the (not sure how it's called) glove-box between the seats and place my mp3 player by the hand brake.
2. you can use the built-in line in for telephone, but the output is only to the front speakers and low sound quality, so - no good.
3.
http://www.css-haupt.de/vectra/zwei/cdr2005_line_in_tel.htmla bit more soldering action, but the best result. it uses the telephone line-in (and not the cd), but sends the audio signal to the main pre-amp of the unit. so you get sound to all speakers and hi/lo/bal/fad tweaks.
I use the first option for over an year now and it's great so far. when I did mine the third option was not yet 'invented', but I'd recommend it. the soldering part is pretty easy, but you must do it carefully, unplug the soldering iron before soldering on the unit's pcb... etc...
good luck
ps. You mean AUX INPUT, not OUTPUT, right?