I am not a pro on this but "15" indicates over charging.
The problem
could be something as simple as a bad voltage regulator which is located on the back of the alternator. Actually easy to change and not too expensive. See this
link for a '90 300E.
I would disconnect the amp first. See whether it is drawing power. If not, take your car to some place and have the charging system checked. If you car needs just a voltage regulator, easy to do.
Haasman