you should've just removed that plug and the small harness. the preamp wires that come out of the factory HU are probably not of a heavy enough gauge to carry a powered signal. i removed that set entirely and just made extensions to the harness.

~Ryan 1999 T-Red SVT Contour 41k Miles #1701 / 2760 B&M, Custom Y-Pipe, Alpine HU, Pioneer 6x8's & 6x9's