Now I don't know if this truly works, but I was just lookin around on the net and just happened to find this.... if you're wanting to ad a sub to your orginal 460 unit this might help-- Now i said might I have Not tried this....
http://www.ehow.com/how_2135105_setup-stock-mach-headunit-system.html?ref=fuel&utm_source=yahoo&utm_medium=s sp&utm_campaign=yssp_art

the link will not open for me.