I replaced my factory cassette HU with a fairly low-end, but quiet, Pioneer CD unit. I also replaced the factory speakers (very important) with Pioneer 3-way, 5x7 speakers which require more power. I ran the HU through the factory booster amp (which, by the way, is speaker-level input) and the system sounds great (no noise). But if there is amp noise on your HU it will be amplified by the factory booster amp. All you need is an adapter to connect to the factory wiring harness at the HU.
The best way to eliminate road noise is to buy four foam speaker enclosures (Crutchfield). They really work wonders. But they are too deep for the front doors and interfere with the window operation if you don't modify they with tape or something. the rear doors are not a problem. I also put some Dynomat on the front doors because that seems to be where most of the road noise comes from. Enjoy.
