auxiliary

With a HU that has a rear aux input, does someone sell an expansion cable with a panel mount mini jack at one end? Or do I have to fabricate my own?

I would like to add an auxiliary input to my 2004 Chevy Cavalier factory radio. It is an AM/FM/CD (single) unit. I know that there are converter boxes out there but here is what I would like to do. Can I make a Y cable out of all of my speaker leads, tie together the +'s and -'s into two leads...