For what it is worth, I call myself a redneck. I grew up in a trailer on the VA-NC line. Like NASCAR, the outdoors, and the like. Speak with a very heavy regional Tidewater/Piedmont blend accent that is specific to the area between Richmond and Raleigh. People that know the region will know what I am talking about. As part of my work, I will either turn up or turn down the accent to either disarm people or let them think I am dumber than what I am. One of my favorite sayings is to end a a conversation with "But what do I know, I am just a dumb redneck." You say it a few times and people will actually start believing you...
Rednecks are a conundrum. They are either dumb and ignorant as hell or some of the most cunning and intelligent people that you will meet. Pretty easy to spot the first group; harder with the second. Sure, you have racism involved with some of the ignorant rednecks but I would not say that it is universal or as prevalent as you would think. For the most part what I have seen are people trying to do the best they can with what they got. They might not have much, but they take pride in what they do have and who they are. I have not seen a redneck get upset about being called a redneck, as long as the caller is not trying to be a shithead about it.
Now here is the funny thing about rednecks: they are everywhere in this country. They might go by different names like country folks, mountain folks, swamp rats, hill jacks, or Floridians (sorry, couldn't resist), but they are everywhere. It is more of a shared cultural experience thing than a region thing. phil-lee has given some really good info about the etymology of the term and the origins from a southern perspective, but as with all things, things evolve over time.
So for all the folks that think that they are being funny or witty about calling somebody a redneck, it is not having the effect that you think it might. And for those rednecks (or other affiliated groups that are cousins to rednecks) that get upset for calling you a redneck, lighten up. Us rednecks have done some pretty redneck shit from time to time.