Can a 500Sl take a larger alternator??

[spinedoc]

I recently hooked up a new head unit in my 1991 500SL, no amps or anything else just a head unit wired up to the speakers directly. Ever since then Im getting some squeeling when I run with the lights on at low speed at night. I am assuming the alternator either needs replacing, or it needs to be larger. I cant believe that the alternator would not handle just an added head unit.

The dealer is looking at it Thurs, but if they say the alternator is good, what are suggestions for a larger alternator that wont mess anything up.