The connector on the iPhone is a stereo connector. It feeds left and right through the base on it's own pin set. I have duplicated the same effect of only having one speaker output by not seating the connector well when I plug it in to the ipod adapter. Look at the connector for foreign objects, lint, whatever and also make sure it plugs in evenly and seats well on both sides. It sounds like something you don't even have to fix by replacing, just find the culprit and make the adjustment.

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~2003 X5 4.4i~