Alternator charging at 12.5V or so

[spdrun]

I killed the battery in my 240D a few weeks ago by leaving the interior light on when the car was parked for a month. Battery wouldn't hold a charge, so I replaced it with a Bosch group 49 unit.

Now I noticed (during a trip back from Boston!) that the alternator is charging a bit, but not enough -- the battery will discharge with the headlights and heater blower on. Max charge voltage seems to be about 12.5-12.6V. Also, the lights flicker quite a bit, and the voltage actually DROPS at higher RPMS (lights get dimmer). Turning off the heater blower and running the engine seems to allow the battery to re-charge enough to make the car able to start.

Do I need a new alternator or regulator? Could a defective battery be causing this symptom? All connections at the battery and ground-strap are tight; I checked. Charge warning light is only on with the engine off, as normal.

Thanks-
-s.