Do you not have a harness that is plug and play with the OEM wiring? Meaning you buy an aftermarket wire harness, splice the small harness with the aftermarket stereo harness, then it plugs right into the cars stereo plug. Most of the wires in these aftermarket harness pigtails are labeled with what they do, so it is very simple to wire up to the new stereos wiring.
If you are trying to splice it into the car's factory harness (by cutting the cars radio plug off), you’ll have to get a proper wiring diagram, and or test the wires to make sure you have the right ones. Much more difficult, and I do not recommend this if you don't know what you're doing.
Perhaps also post photos of your wiring you are working with which might help a little.