In order to install automatic temp control, you need to change both the dash harness and the lower ATC harness, replace the blower resistor with the contol module, and replace the head unit with the ATC one.
It is "possible" to add/delete certain wiring in the dash, keeping the existing harness, but i'd advise against it, since it is a complicated swap, and you still need to pull the dash.
Depending on the year of your car, I have "kits" available that include all the parts necessary, along with full instructions.