newer models need to disconnect the module GTAlpine mentions above to disable DRL, but auto-on is still enabled. To disable auto-on, you need to add a resistor to the light sensor in the center of the dash to make the computer think it is always sunny.
older models just needed to pull the fuse, which took care of both DRL and auto-on.
as others have said, see backinblack's site for specific details.
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'99 GTP, L36 bottom, XP cam, Headers, 3.29 gears, FWI, Powrtuner