97 Sonata

Replaced Alternator, Charging ok, but when my Grandson turns on the lights, and another item there is a squeel, only when this happens. Same thing with the previous used alternators, this one is rebuilt from Autozone. Help please, could it be an Idler??????

  I would be inlined to check earthing points, say from battery to body and engine. The sqeeling could be an indication of a heavy load on the alternator. If you can, on your model: Put your headlights fully on then start the engine and see what happens.
If your battery is fully charged and you have a problem turning the engine over and thelights go out, I would check out the continuity from the battery terminals.