Noise Problem

Hi all, as you are about to find out I only know enough to be dangerous. I listen to music at work by hooking up a boombox to the battery on the equipment I use. When the boombox died (an old JVC with great bass) I tore out all the guts and just stuck a car amp inside, and use an MP3 player for source, everything works great except the battery in the MP3 player won't always last the shift, so I also want to hook it to the big battery powering the boombox. I got a cigarette lighter adapter for the MP3 player and hooked it in parallel to the same battery connection that the car amp is running off of, and when it's all hooked together I get a lot of interference type noise. Everything works great seperately (headphones in MP3 player or alternate source to amp all sound fine) but hooked all together it's got this noise problem. What can I do about it?