just as the term "redneck" literally meaning the neck of a person is reddish because of working all day under the sun. So usually the term red neck is for people who work outside such as plantation worker, farmer, fisherman, construction worker. Sometimes it is used to group people without manners. When you greet a female stranger with "honey", "sweet lips", "blondie"....., usually the person will be labelled a redneck.
FYI, it is not true that all people without education have no manners, but most of the time, they do those rude things.