Charging sytem problem, too

My daughter's 93 Protege, which is just a few miles short of being a paperweight, stopped charging the battery. It only put out 11.5 to 12v, and would go down to 9v when lights, defroster, etc. were run. Nattery was 2 yrs old (but cheap). Bought a new alternator, had the same problem. They Autozone guy said my battery was bad, got a new one (somewhat warrantied). Same problem. Next they told me the alternator was bad. Got another one-same problem. I'll ask here before I install the next alternator (btw, the first two replacements bench tested ok after I returned them). The battery light lights when the voltage drops, making me think that the wire between it and the alternator is ok, and the other two wires have constant voltage. I wire stuff for a living and have all the meters I need, but cars are confusing to me. Any ideas? Thanks!