You can cut off the OEM harness, and then spend hours trying to figure out what wire does what....do you even have the proper testing tools to do this?
If you buy a harness, all you need to do is match the colors, can't get any easier than that. And trust me, someday you'll need to remove the radio for whatever reason, and it 1000X easier to just unplug a harness than to cut all the wires, then reattatch them all over again. Then the money you'll need to spend on butt connectors could've gone towards a harness, and you'll have many hours of your life back.
It's a good investment. If you don't want to spring for a Metra harness (the best), they may have a cheaper one at WalMart, they carry Scosche harnesses.