My Nexus 6P thinks my car line-in is a headset. It shows some notification about using a headset when I plug it in. The standard double ended 3.5mm jack cable doesn't have a microphone ring so I'm not sure what's causing this behaviour. The car also doesn't support using a mic over this connection.
This behaviour stops the microphone on the phone being used because it is expecting the mic on the non-existent headset to be used. Wifi and Bluetooth turned off and not connected to USB. It doesn't do this with any other 3.5mm connection, such as to normal headphones or into a stereo.
Is there a way around this? What is causing this?