Car is a 1980 240DL.
Once again the alternator is causing grief.

Was sitting idling on my friend's driveway at around midnight when my output drops from 13.8V down all the way to 12.2V. The alternator idiot light starts to illuminate.
I shut off my car, try to restart, and all i hear is a click from the starter.

Each time i turn the ignition, the starter makes a single click, but the engine doesn't crank.

Jump start does nothing. Starter is new and is about 1 month old, and tapping it does nothing. So i had to resort to bump-starting my car ( its a standard) and limping it home at 12 volts with the AMP idiot light on the whole way home.

My car is sitting on my driveway where i'm going to leave it for the next week or so cause i dont have time to mess with it.

Basically, before I start doing anything, what are some possible culprits?

I was thinking either loose or worn drivebelt, or worn brushes or voltage regulator. Do those 2 sound like possible problems that would cause a no start?

What bugs me is that the car won't even crank over. Even with a faulty alternator, shouldn't the car still crank?

Any other possible problems you think i should look for?

Need some input from you guys.
Thanks in advance.