Problem with Stereo

I bought a JVC cd/mp3 player from my friend. So, we took out the factory radio, and installed the new one into my van...It sounded fine with the other unit, but now, it sounds terrible. My speakers make annoying clicking/hissing noises... I know a few of the speakers are blown, but this sound is coming from my good speakers too. I bought my van from my aunt, and I know she had the same problem. So, what's the deal, and how do we fix it?

Try a ground loop isolator