Have you cleaned the battery cables and terminals? As a last resort you could go around and try cleaning all of the grounds up. With that said, if the battery and regulator are new and the car is still dying then really there is only the alternator left. These alternators were pretty weak when new, especially when you consider the amount of electric features these cars come equipped with. If you are planning to upgrade the headlights or stereo system in the future, I highly recommend that you upgrade to a more powerful unit.

__________________
1989 300CE