rift, go on crutchfield and there should be one for your HU.. otherwise as gonzo said, there are definitely cheap ways to create an AUX on that HU.
sterling, go to this thread:
http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=38;t=003536
it is possible to create an AUX in for your RF 6 disk head unit.
whitman