Electrical interference noise in car speaker?

My car has a factory installed stereo system. I also had an ipod adapter installed (which fools the stereo into thinking that an ipod is a 10 disc changer). The car stereo sounds great but when I switch to the ipod, I hear a screeching sound in the background which tracks the accelerator. The more I hit the gas pedal, the higher the frequency of the sound. So, it is clearly related. How can I fix this? The car dealership says it cannot be fixed.