Yes you do need a wiring harness. They make them to match almost all makes and models of cars. You might also need an antenna adapter (most gm cars and trucks do). Stop bye your local car audio shop and they should be able to get you the parts you need to match your car.
get the harness(es) for your make, model, and year of car. connect the harnesses to your new stereo's wiring, and all you need to do is plug the new head unit into the car's factory plugs. No need to cut any wires, and if you sell the car later, you can re0install the old radio effortlessly. well worth the $10