I have the same problem in my Tacoma pickup truck. With Bluetooth, the volume setting is limited to 15. Turning up the knob on the car radio momentarily brings the volume to a useful level, but some software somewhere in the radio or the iPhone detects that I can hear the program and immediately turns the volume down again. Music settings for volume limit, EQ, and sound check are all OFF.
I solved the problem by wired connection of the iPhone to the car radio USB port. But it would be nice to restore the blue tooth functionality. It used to work great. But now I don't have my flash drive with all the audio books for long road trips and there is one more wire cluttering my cockpit (besides ham radio, GPS, and radar warning receiver).