I leave it on permanently. However, I would first check if your car switches off the 12v connections. Because certain models leave them permanently under power and then it could be that the USB charger discharges the car battery. (if you have other USB devices connected. Only the Bluetooth adapter itself doesn't matter, it needs extremely little power, then you can leave the charger+adapter connected without any problems).

The adapter lights up white permanently when a mobile phone is connected. When a call comes in, it starts flashing white like crazy. So you can see it. Unfortunately, you can't switch automatically when you listen to music from a USB stick or something like that, because the adapter is only connected to the car via the headphone jack, so you can't send any control signals. If you play music via your mobile phone, however, it works perfectly, the music is paused and the call is put through. You can also answer the call directly via the adapter.