alternator or regulator?

I jsut bought an 89 300E and had it looked at by a mechanic who said it needed a new alternator as it was only charging to 11.5 volts. When I took the car back to the dealership for the repair today (they were going to cover it) they instead replaced the regulator (said it was cracked)

My question, could that solve the problem? I know the repair for the reg. was less than half of the price for the alt.
so it has left me with a question in my mind.

Any feedback????

Also, they fixed the tail lights and the car alarm went off when I went to start the car and wouldn't go off until I went and put the key in the door????? what was that???

------------------
proud owner of 300E