ummm.. probaly a blue one, lol, thats what it usually is.. a multimeter helps tons on stuff liek this, or if u have a 12v light bulb or 12v operated "object" u can do some trial and error, im going to look and see if i can find it right now, be back in a minute..
edit- im not sure, im thinking possibly yellow or pink, but best/easiest i think is to "test" the wires with something.. and were talking about the stock HU right? cause it will be blue/white on most aftermarkets..