iPhone thinks Bluetooth FM transmitter in car is a set of headphones Keeps Turning Volume Down

I use FM Transmitters in my cars to play music from my iPhone. The problem is that the iPhone thinks its a set of headphones and keeps turning the volume down. There is a volume adjustment on one of the BT devices the other has none. The one with the volume adjustment does not compensate for the iPhone turning down the volume. It's a real pain, any ideas how to fix it?

Thanks

Guy