As others have stated, you have a "wired" FM modulator. I installed one of these in my wife's SLK (no longer have) because there were no other option available for the radio she had. It seemed to work fine except for the fact that all control of the iPod had to be thru the iPod. What I did notice though was that the FM modulator did interfere with the reception of other radio stations, so I ended up installing a swith to where I could turn it off when not in use - this resolved the interference issue.

Although DICE is not perfect (it has its quirks), it is a much better solution than what you are trying to do.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!