The last Dual deck I had was the XD 7500, and the input was actually plugged into the input(yellow) RCAs on the back of the head unit, and they called it the iPlug. On my HU it was plugged into the yellow RCA jacks, and the RCA outs was actually in with the rest of all the speaker, and power wires. From what I've looked at on many sites, yours has the same thing.
If you don't have the iPlug, you can simply go to radioshack or a similar store, I've even seen them at walmart, and buy an RCA to 3.5mm(headphone) phono jack. I think they usually run about $5($USD). Make sure you buy one thats stereo, which will have two RCA female ends, going into one 3.5mm Phono Jack