If you have volume but cannot control it, this is a common problem. The volume is optical so it may just have dust in it, but I have had zero luck cleaning them. The only solution I know is to replace the whole head unit.
If you have an HK factory stereo with no volume at all, and the four channels went out one at a time, the amp in the trunk has failed. Another common problem. The amp is on the driver side above the wheel well. You have to remove the driver side rear deck speaker to get the trunk lining off without tearing it. The screws that hold in the speaker are beneath the speaker grill in the rear deck. Good luck getting a 15-year-old speaker grill up without breaking the snaps. Slide a putty knive in from the sides to try to release them, but they will break anyway.
If the amp has failed, you can just bypass the amp but you will have no subs or tweeters. I did that while looking for a good deal on a junkyard amp. You can buy a new HK amp from BMW but they are prohibitively expensive. You could buy an alternate brand amp and add crossover circuits to power the subs and tweeters if you are really handy with electronics.
If you have an HK system in your BMW and you replace the head unit, use the antenna power wire from the new stereo as the 'on' signal for your HK amp in the trunk. Your FM may not work quite as well because the BMW head unit communicates with the antenna amplifier. Also, if you have a BMW CD changer in your trunk, no other head unit will communicate with that other than the original BMW head unit.
Other than the CD changer and the antenna amplifier, the wiring adapter that you can get from Crutchfield or from Amazon will make the stereo head unit replacement plug-and-play.
You may need a special five-sided wrench to get the old stereo out. You can buy that wrench on line if you can't force some other tool into those two cap screws that hold the stereo in.
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