Sure...
I have a '02 Jimmy. My stock deck is a Delco. The deck was designed to be able to control a CD changer. But I don't have a CD changer. So what I did, was buy a cable that "emulates" the CD changer interface, but actually has 2 RCA jacks at the other end, so you can hook up basically anything in the world that has RCA outputs. Does that make sense?
Check out this site :
Car Installer Parts Install it Yourself - Car Stereo Wires, Tools, Harnesses, Dash Kits, Car Play
They have tons of adapters. But being as your deck is aftermarket, you probably won't see anything there.
What is the make/model of your deck? Try calling a local car stereo place and see if they know what they are talking about. If so, ask them to look for one for you.
The price might be a little steep for you. Seeing as you already spent $200 on an aftermarket deck, you might not want to spend another $80 on an adapter. Maybe buy a new deck? For me, it was worth it, because I converted my stock CD deck that works great otherwise. I haven't looked at any decks lately, but you cold probably get a brand new deck for little more than $80 that will have an AUX input, probably in the front of it for easy accessibility. I had to extract my deck, run wires through the dash, and I have 2 RCA jacks hanging out the bottom of my dash. I will try to take a picture and get it up here to show you.
Hope that helps a little....