Agreed. It will depend on the specific device, of course, and what it was designed for. I'll give you two examples from items I use.
This first is a little older because I used it in my last car. It was a hands-free cell phone device made by Motorola that clipped onto the sun visor. It used bluetooth to connect with my bluetooth-enabled cell phone. When turned on, that device took over sending and receiving phone calls via the cell. It had its own speaker and mic. You could use voice commands to make calls, and it answered incoming calls. It also had an optional feature to send its audio output to the car FM radio instead of to its own internal speaker.But that was a low-power FM transmitter, not part of bluetooth. My new car has bluetooth hands-free built in, so I use that and have passed on my old device to my son-in-law for an older car.
I purchased last year an ODBII reader. It plugs into the diagnostic connector in the car that you might normally use with one of those $200 Reader boxes. But this is not a whole reader, and it costs only $30. It gets power from the car diagnostic socket, and uses Bluetooth to connect to my cell phone. Then on the phone I downloaded an app ($5) called Torque that talks to that tiny reader module. The module merely is the communication link between my car's computer diagnostic system and the app on my phone. Together the app can show me all the diagnostic codes, then use the internet (by the phone's WiFi access to our home network) to look up exactly what they mean with suggestions for actions, just as a dedicated hand-held box would do. The app also can display in real time by customized graphs a huge number of car data like GPS Position, a map of where you have just traveled, RPM, fuel consumption, speed, vacuum signals, engine temps, etc., etc.
So all of these devices - the older stand-alone hands-free device, the new built-in hands-free phone system and the ODBII reader module - used bluetooth to communicate with my cell phone in a bidirectional data transfer mode. Now, each bluetooth device can connect and communicate with only one other bluetooth device at a time. Since I now have two devices that use my cell phone, I keep it set to work with my car's hands-free phone system by default, and it is activated very time I start the car and its system searches for my phone. But if I want to check out the car's diagnostics, I plug in that module and use my phone to get it to talk to that device until I'm done. Then I switch it back to using the hands-free system. My phone keeps both devices in its memory so I can switch easily by a menu choice.
I'll just note that there are many devices on the market that can play audio from a player loaded with MP3 files though your car stereo radio system. Many of these are NOT using bluetooth. They simply include a small low-power FM radio transmitter and you just tune your radio to that frequency. That's a one-way data transfer (actually, an analog radio signal). Since ALL car radios today include FM receivers, that is a more "universal" technology for one-way communication that trying to use bluetooth, which many car radios do not have.
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