The RF system includes a head unit (Panasonic) mounted in the dash. The head unit is connected to an amplifer (mine had a Clarion sticker) mounted under the driver's side dash up on the firewall. The amp drives 8 (KC) or 10 (CC) speakers. In a CC, the speakers are: two tweeters and a center channel on the dash, one 6x9 in each front door, a tweeter in each rear door, a speaker in each rear door, and a sub in a plastic box under the driver's seat. My speakers had the indicia "Clarion 2 ohm" and "35W".
To swap out the HU and drive the OEM speakers with a new HU, you need to bypass the RF amp. For many other vehicles having OEM stereos with external amplifiers you can buy what is known as a "bypass harness". This basically connects the speaker outs of your new HU to the speaker wiring of the factory harness at a point downstream of the OEM amplifier. I haven't seen a "bypass harness" for Nissan's RF system. However, Eakes has solved this problem and has previously posted on this site instructions for jumpering around the RF amp using wires and Molex pins. I think you can probably find this by using the search function for posts by Eakes.
In addition to jumpering around the RF amp, you would need a pigtail/connector adapter that would connect the harness that comes with your new HU to the factory harness connectors located behind the dash. That pigtail/connector adapter is available under the designation Scosche NN03B. Crutchfield also sells an adapter as described at this link:
http://www.crutchfield.com/S-bVFsSsa...an+1995+%26+up
And also the replacement dash bezel mentioned by Eakes:
http://www.courtesyparts.com/Merchan...de=68260-A60CC
Now when you completed all of this, you would have a new HU driving crappy 2 ohm speakers and sub. Depending on the characteristis of the new HU, it might not work so well with the 2 ohm speakers.