Look, don't settle for a band aid fix. Just get a new head unit with auxillary inputs. I would also recommend getting a head unit that is XM or Sirius satellite radio ready. You may not want it now but you will, I guarantee it. You can pick up units starting at $150.

I know you guys are fighting with the reasoning of buying a new aftermarket head unit since you just purchased the upgraded stereo in the Xterra. Just do it. Or you can troll the boards for months and years trying to figure out a way to magically create auxillary inputs in your Rockford Fosgate factory head unit.