I have replaced the alternater & belt on my wife's 1980 cutlass supreme and it will still squeal if the headlights or heater are running especially driving low speeds. It has the 260 & I think someone changed the brackets. Does anybody have pics of what their supposed to look like or ideas?

 

Check for loose belts. If the alt belt is tight, check to see if there is an inner belt driving the alt belt. It may be the loose one.

 

maybe you should place this thread under electrical