What does the dictionary say?
Webster's Collegiate, 1996 Tenth Edition: "1. a white member of the Southern rural laboring class. 2. a person whose behavior and opinions are similar to those attributed to rednecks- often used disparagingly."

Oxford English Dictionary, 1987 supplement: 1a. "a member of the white rural labouring class of the Southern States; one whose attitudes are considered characteristic of this class: freq. a reactionary."

First known use- 1830, by Royall, "Southern Tour." "This may be ascribed to the Red Necks, a name bestowed upon the Presbyterians at Fayetteville" (AK).
1893, Shands, "Some Peculiarities of Speech in Mississippi," p. 53, Red-neck. "A name applied by the better class of people to the poorer inhabitants of the rural districts."
1904, "Dialect Notes, II," p. 420. "An uncouth countryman. The hill-billies came from the hills, the rednecks from the swamps."
1913, J. Davis, "Life and Speeches, iii, 42. "If you red-necks or hill billies ever come to Little Rock [AK] be sure to come and see me- come to my house."

1936, Faulkner, "Absalom! Absalom!" p. 122. "Rich and poor, aristocrat and redneck."
1959, The Times Literary Supplement, 28 Aug. (London). "The ugly faces, and, under prompting or provocation, the uglier actions of a handful of red-necks, crackers, tar-heels and other poor white trash here and there in the South." [Note: North Carolina is the Tar Heel State. Some politicians would call Georgia (the Peach State) the Cracker State, but seemingly little support for this]
1960, The Spectator (London) "The old patrician families who are opposed to the graft, blackmail and demagogy by which the Boss, the tribune of the rednecks, keeps himself in power."
Many more.

In 1976, Jimmie Carter, who just was given the Nobel Peace Prize, called himself "basically a redneck."

S. Af. Rooinek. First applied to some merinos brought to the Free State and marked with a red brand on the neck. Later came to mean "another bloody English immigrant."