Try repositioning the stereo wires. You might be living close to an AM transmitter tower and the existing stereo wires, or trickle charger could be acting as an antenna for the AM frequency they are blasting.
It can also help to make sure that every piece of electronics involved with the stereo is grounded to the same point.
If that doesn't work, a third option may be to hunt around for some RF chokes that can clamp right on the wiring for the line feed from the head unit to the speakers. These can usually be found at the larger electronics stores.