Hey I think I have the same exact headunit in my Avalanche C4. The good news is you can do it, the bad news is you need to buy this part, those inputs only activate with a Sony CD changer or MD player here is what you need:
Auxiliary Input Adapter for Unilink Receivers
You can find it here, and they also have the instruction for installing it, that you may find helpful, the unit does alot. http://www.crutchfield.com/S-L5PY09IUPIA/cgi-bin/ProdView.asp?i=158XA300&search=xa%2D300
Just be carefull how you adjust the volume on your ipod or you will overload the input, if you have the ablity to use a line out even better, some chargers have this feature.
Also there is a pretty slick waterproof wire remote for this unit, although a little pricy.
Hope this help..........Cheers!