I don't think you can do what you want to do with that extra RCA. Your S/PDIF is output, not input, so that is out too. I think your best bet is the "CD-IN" connector on the motherboard. Check out this thread:
AUX Input options
Look at the 6th post. You can buy (or make) an adapter to turn that CD-IN connector into a 3.5mm line in jack. I believe it will show up in the windows volume control as the "CD Player" slider.