You can use a couple of RCA "Y" cables and a couple of these remote-controlled electronic switches to switch your RCAs from the Ipod to the head unit (they're available for about $15):

http://www.scosche.com/scosche/electronic/US2B.asp

Basically you'd use the "Y" cable to split your IPOD's two outputs into 4. Then you plug those 4 outputs into the two switches, and the 4 outputs from the head unit into the two switches, and the output from the switches into the amps.

The only disadvantage to this is that you won't be able to fade the audio from the front to the rear when you're listening with the IPOD (unless you find a fader control. I'm sure someone makes them. I'm not sure it's worth the effort to track it down.)