low music volume when playing music via bluetooth

When I play music on my iPhone 6 Plus via a Bluetooth connection with my car stereo, and the volume on my car radio is already at an appropriate sound level, the music played from the iPhone is at an almost inaudible level on the car stereo speakers. So I crank the volume way up to listen to my music, but I get blasted by extremely high volume sound if I switch back to the car radio. This happens on my current vehicle, a 2015 Fiat 500. The same issue was a problem with my previous car, a 2012 Toyota Prius. Is there a real fix for this issue?