Stop high frequency noise from iPhone dock's line out in car adapter
You're discovering the joys of noisy electrical grids in cars.


  Do I need an amplifier?


Nope. You need a filter.

And filtering noisy car electricity is...tricky. You can't just create new, separate paths to ground because there aren't a lot of choices for where ground is in a car.

You can try putting a simple choke filter on the line between your player and the input to the stereo. That's the cheapest and easiest way to try and fix the problem. Something like this is sufficient.

If that doesn't cut the noisy you need to filter the power. Finding one of those car DC to AC inverters with a built-in USB port is likely your best bet. Something like this is what I'd suggest you try if the choke doesn't help.