I bought bluetooth-to-jack audio receiver by cellularline, which is announced to be compatible with any device supporting bluetooth 1.1 or higher.
Yet, neither my son (FP2 with /e/) nor I (FP2 with latest LineageOS-for-microg) can connect. On the first pairing, there is a confirmation dialog for pairing, then it keeps disconnecting/reconnecting/disconnecting/reconnecting… forever without pause, and VLC audio is never sent to the device.
At first, I did not understand where the cache had to be cleared, and then I found it: In Applications → All applications → Display system processes → Bluetooth.
I cleared all data, which includes cache data, and rebooted the phone.
But unfortunately, that did not allow me to attach the Bluetooth audio receiver.
I also tried deselecting phone calls and contacts, keeping only multimedia in the audio receiver Bluetooth properties. That did not help either.
At home, there are no other phone models to test with. However, my wife’s FP2 still has the official Open OS installed, but it behaves the same, whether or not I select phone calls and contacts in the receiver properties. Strangely, her FP2 does not have “Bluetooth” in the list of system processes (it does have “Bluetooth MIDI Service”), even though Bluetooth is working, and we sometimes use it to share files.
Well, your device as stated in its technical specs works with bluetooth 4.2 while the FP2 only offers BT4.0 LE, I guess that the cause for your troubles.
I’m using this type at home which works fine with my FP2.
It uses BT3.0. In general it could easily be made applicable for outdoor/car use as the power supply is giving out 5V DC at 0,15mA.
But you would need a matching power adapter let’s say from a 12V DC cigarette lighter socket using a general purpose USB converter. The cable itself then would have to meet standard USB to a barrel plug (smaller than 5,5/2,1mm type), my measure 4,0/1,7 mm.
Additionally an audio cable is needed with a 3,5mm stereo plug or red/white RCA on both ends, this converter offers both options.