The Sat Nav has its own amplifier on-board, your most likely culprit I believe will be the Radio unit itself.
Look on ebay at the BMW BM24 which is far better quality, and a straight swap.
I'm sure one of the many electronic members will be along to assist in your query soon.
"My lovely car now sold onto a very happy new owner.
I still love this marque and I will still be around, preferred selling to breaking, as a great runner and performer"