I have a friend that had a broken original BMW radio (525tds), the volume
always went to the maximum on the left speaker.

The problem is that a storage IC in the radio was broken, few bits in the
I2C always returned 1, so that causes the radio to go max volume and on the
left speaker.

The IC is located inside the radio on the main baord. The solution was
solder out the old IC (24C16), read it out with an IC programmer, take a new
24C16 and program this with the old values. Solder it back again. Everything
worked ok again.

Hope this helps when one of you has similar problems!