Separate volume control with multiple Bluetooth devices?

Is there a way of controlling the relative volume sent when an iPhone is paired with multiple Bluetooth devices?

I have a 2005 car with an early Bluetooth version that supports phone calls, but not music. So, I bought an after market Bluetooth device that transmits the signal from the iPhone via FM radio. Somewhat amazingly, it mostly works in that I can pair both devices, and phone calls are routed to the built-in car system (with answer/hang up on the steering wheel), and all other audio (e.g. music, navigation) is sent to the FM radio. The problem is that the volume levels are wildly different. I need to blast my radio to the max.

More generally, if you pair an iPhone with multiple Bluetooth devices, is there a way of controlling the volume for the devices separately? Can you intentionally send phone calls to an earpiece and music to a speaker, or is it just a fluke that mine works because of the older Bluetooth versions?