Not sure if there is anything specific without looking on tinterweb but This idea "may" work. You could wire up a small amp ( 4 channel if your going to run 4 speakers ) with a manual on/off switch. To connect the ipod to it, you would need a decent lead that at one end connects to your ipod ( 3.5mm i think , ) and at the other end connects to the amp via RCA leads ( red and white ). Mount the amp out of site, say in the boot and run the rca lead from the amp to your preferred point in the front of your vehicle for the ipod.
I have never tried this method as I use a head unit to connect an ipod. I do however, use this method at to connect my Ipad to my studio gear at home, which also has my synths and guitar going through the amp as well.
Cheers