Easy. Really all you need is a RCA to Mini (3.5mm) cable for the signal between the iPod and the amp. The Mini jack goes into the iPod (just like your headphones) and then the RCA ends go into the input side of the amp. You will need to adjust your amp gain and your volume on the iPod until you find the volume range that you prefer. I think the RCA to Mini cables are less than $10.00 for about a 16 ft cable. I have attached a picture for your reference of what a RCA to Mini cable looks like.
Exactly how I would do it. I would have a couple of reservations.
In reality I think that you will really miss a volume knob in a car installation. I know fooling with the volume on an iPod (for me) is slightly cumbersome, and while driving could have you with your eyes off the road for longer than you realize.
If you are handy with a solder gun I would imagine that you could rig up some kind of volume knob (rheostat) on the RCA cable.
The other thing, an iPod has a really tiny amp, thus even the small amount of line level volume it puts out can easily distort above about the halfway mark on the iPod volume. Its not easy to distinguish with earbuds, but having the small amount of distortion it is putting out amplified could wind up sounding horrible.