Yes there’s a line in but the volume level on it is terrible, really quiet
I’m sorry if this is stating the blindingly obvious but I have to ask, did you try turning up the volume on the phone? You’re not providing a Line level source (which would be fixed), you’re providing one intended for headphones.
I was hoping that if I could use the dongle I could still use the steering wheel controls for skipping/restarting tracks but I’m assuming that wouldn’t be possible with the FM transmitter.
This is the same problem as previously. The stereo would have to support the profile that provided this (AVRCP this time, unless it’s been superseded by something else since I last looked). Bluetooth is kinda complicated. It isn’t ‘just’ Bluetooth but a suite of optional profiles that provide different functionality, there’s dozens of them. It should be kind of self-evident as to why really, it’d be a nonsense to have a printing protocol connected to a headset.
When pairing a phone to a car, you might see this in action on either the phone or the dashboard. Hands-free and Headset are two separate profiles, the former takes care of phone stuff like dialling, the latter carries the voice.