I know you want more control than this will give you, but we have a fm transmitter that we attach to a i pod mini, you would only be able to control the volume though. You could just put it on shuffle maybe? Also I would suggest you get a car charger adapter for the i pod also, without ours being plugged in to the car the signal is only strong enough to produce mono sounds.
I am sure this is not the solution you were looking for, but it is a low cost alternative.
good luck
taf