Line in bavaria c2

[mkmini]

Hi!

I found in archive this post http://e30-talk.com/topic/67919-aux-in-anschlussklinkenstecker-line-out-an-originalen-bmw-radios-nachr%C3%BCsten/?hl=%2Bumr%C3%BCstung+%2Beines+%2Bbusiness+%2Bcd+%2Bauf+%2Baux-in+%2Bzum+%2Banschluss+%2Beines+%2Bmp3players%3A&do=findComment&comment=872938

I have bavcaria c2 - and am trying to make propper line in.
Soldered L+, R+ and - as in the pictures, but I have still one problem - there is a little mix mp3+radio.

Is it because the - is soldered to the corpuss of radio - and it should be instead soldered to point "mass fur cassete" ?

I don't understand german at all, so if You are answering could You please do it in english :)

Maybe some of You have bavaria c2 for spares, mine is missing tone regulator button, and I have made a crack into display

[SunnyDriver]

do you also have put put a electrolytic capacitor in between each cable for R+ and L+?  (C = 2,2µF/63Volts each)