I wanted to upgrade my Classic Prius sound system, but unfortunately the onboard sound system is integrated with the hybrid system and I did not want to mess with that. Instead, I just installed a separate sound system. I connected a cellphone (can play mp3 files) to an amplifier. The amplifier is a 12-volt unit that is compatible with the vehicle lighter power and works out of the box with a basic connector (no power converter). The cellphone is connected to the amplifier using a 3.5mm cable. The amplifier is then connected to the two speakers I installed via standard speaker wire. Everything works fine. The only problem is that I get a ground loop hiss in the audio if I am charging the phone using vehicle lighter power. If I am just using the phone without charging it, there is no problem with the sound. But I would like to charge the phone at the same time.

How to fix the ground loop hiss? I tried ferrite beads around both the 3.5mm wire AND the USB cable that charges the phone, but it had no effect.