Yeah, I put an 02 changer in my 00. The 4-ohm/2-ohm speaker refers to the resistance they provide. The 02+ head units only have enough power to drive the lower-resistance 2-ohm speakers, and when hooked up to 4-ohm speakers they just won't push hard enough.
No fix, either replace the speakers, or get a line-level converter and an external amp. Or just get a non-OEM head unit in the first place.
Oh, and Nissan did have an external changer available for the 00 (SE only) that I see on Ebay now and then.
Brent