If it's a DEH-P model, then you probably need the AUX-IN adapter.
When you say there are 2xRCA, does that mean two RCA plugs, or two PAIRS of RCA plugs?
Either way, I can almost guarantee you those are outputs. If you have a DEH-P, then you have an IP-BUS connector on the back which should be a blue connector with a bunch of pins. You either need to spend $30 on the adapter for it, or make your own. (
http://www.gutterslide.com/ip-bus_hack/)