Car is an 83 240D.

I welded on the floorpans with the alternator and battery connections disconnected. Successfully. Yesterday, when I was headed home, the charge, brake, and pad thickness lights started dimly lighting up.

Sure enough, the alternator is only charging at 12V at idle, 12.2V at higher RPMs. Would this be likely to be the regulator or the alternator itself? A.C. voltage at the battery is near zero (0.1V) , so it's unlikely to be the diode pack, right?

Did the welding somehow damage the alternator? Lastly, there's a single connector on the alternator separate from the regulator and three-pin main connector. There's a wire on it that seems to have been connected to a black box on the back of the alternator, but has broken off. Is this just a filter capacitor or something that would cause the alt not to charge?

You aren't charging at all.

It's probably the regulator. Last year, I was doing a lot of welding on my 190. I've been told that the regulator can burn out if stray current from the welder finds it's way to the regulator. I didn't believe this, but sure enough, my regulator came up bad.