Just a matter of interest I have finally changed the head unit in the 525i using the info off this site and I have a couple of points. Pin 10 is listed as 12 volt switched but is only for speed sensitive volume control so is not needed pin 5 is for ignition pin 13 is illumination (dash lights) on the head unit I have this dims the stereo when the lights are turned on. Any way I thought I would post this for people who are doing the same thing as I looked in the bently book to work things out I don't know if this has been posted before. I installed an Alpine CDE-103 EBT head unit and the fitting went well and I couldn't get an E34 iso adapter so went with an E39 which has the same plug but I had to change the wire positions on the plug match the E34 I soldered the wires that came with the unit to the plug but if I did it again I would cut them shorter since room behind the stereo is tight and I had to push wires around to get the unit to lock in but it looks great and fits well. The alpine has 4 colour display incl red and has bass and treble center adjustments and bass bandwidth adjustment so it can be tweeked to give even sound through the cabin. Just for people who might like to know