Quote:I can't really connect the amp to the battery because I want the stereo to turn on when I turn the key.
You can use a relay for this, but that might not solve your problem.
If your comfortable with a soldering iron then try this:
Use shielded wire as short as possible. I use 18 guage mic wire a lot for power wire, but usually never fuse it above 5 amps. Only connect the shield at one end.
You can also solder a cap across the power wires close to the amp/stereo. Even better are 2 caps. A large electrolytic say 1000uF or larger in parallel with a small ceramic .1uF or larger. (The small ceramic responds to high frequencies better). Even better is a coil in series before the caps. Something in the mH range or larger. They work together to make a great low pass filter.
Very simple but not as effective are the clamp on ferrite shields. You see these all the time on modern electronics close to the connector. You can buy them at radio shack.
It may be as simple as rerouting your power wire where it does not pick up noise as easy. Also keep it as short as possible.
Try a different ground point and make sure you have a good connection. You could be picking up noise through the frame.
It could be a mechanical vibration problem with your electronics, Amp, mp3 player, whatever your using. Try isolating them with some foam.
Also make sure all your connections are good and protected from vibration.