OK. Done some more diagnostics

Disconnected the red exciter wire and grounded to the chassis. When the key was in the run position the battery light came on - so the red wire is OK. So I think this narrows it to the alternator, brushes or ground.

I ended up removing the alternator. Ground wire seems OK - undid the screw that connects the ground to the alternator bracket and cleaned it up. Did a continuity test from the ground wire to chassis and seems good.

With the alternator off I could get a better look at the brush assembly. I struggled to instal the brush unit with the alternator on the car but it looked OK with the alternator off the car. Removed it again and studied the brush contact points. One was a nice copper color. The other had 1/4 copper color, 1/4 darker/black, 1/4 copper color, 1/4 blackish - this as the one that was completely worn down. Could this be the problem? Tried to clean up didn't make a difference.Can anything be done to this - clean it up with sandpaper while spinning the rotor?

Took to Advanced Auto to get it tested but they didn't had a wide enough bracket to mount it to their machine ! So it seems like it the alternator but always like proof positive before throwing parts at the problem. Any ofter diagnostics that can be done on the alternator itself?

If I have to replace the alternator, whats a good brand/source that is cost effective?