you will find a couple of other threads and posts about this, but when installing an aftermarket head unit into stock Rockford Fosgate setup for a 02-04 SE xterra, you will need to bypass the amp. All of the speakers are wired into the factory RF amplifier. Chances are, only a stock RF head unit will interface with this amp (that's why you are getting no sound.) You will need to pin-out all of the speakers on the RF amplifier harness to find the speaker connections, then basically reroute them into the pins on the head unit harness. PITA for sure. another alternative is an FM modulator where you'll lose about half of the sound quality.... or you can mod your head unit.
(see next post) -waltman