You'll need to visit a site like http://www.the12volt.com and get your stock vehicle wiring colors and what they mean. From there it's just a basic hardwire, matching up to the colors on your aftermarket head unit harness (the manual will tell you what wire is what if it is not printed on the wire ... do not immediately match up the colors, a dark green wire in the vehicle may be a positive for a speaker, and something totally different on the aftermarket head units harness [bad color example, but you get the point])
If your stock harness isn't cut, definitely do as the above guys suggested ... grab a $6 harness and use it.