The power supplies in these small amplifiers tend to interfere greatly with FM reception. Here is a great article discussing that very thing.
There are three things that can be done to combat it (a little.)
and a fourth option that seems to be the most appealing to the majority of riders.
One option is to relocate the amplifier as far as possible from the radio chassis and antenna cabling.
Another option is to use a whip style antenna (sticking up in the air) at the back of the bike. (This gives us the biggest regain of FM reception).
A third option is to run an antenna along side of the bike frame running as far as possible from the front fairing towards the rear of the bike. This antenna would be a good option for a job like this.
Your fourth option for when those two are still insufficient, is to download to your phone, a free app called iHeart Radio.
It allows you to listen to every music station, but without the traditional distance limits of FM antennas (You can listen to your favorite radio station all the way across the country.) Now just Bluetooth stream from this app to your radio.
---This option is fantastic but is dependent on your cellular reception on your phone.