You'll need a Nissan wire harness, and possibly an antenna adapter. If your stock radio says "diversity" on the front then you'll need the antenna adapter. If there's a factory pocket under the stock radio, you won't need a mounting kit, although the install may look better if you use one. If the factory radio is a double-din system, you'll need a kit.
If the Maxima is equipped with a Bose audio system, you'll need a premium sound adapter or you'll need to do some rewiring.