set Android 6.0 default USB config to MTP rather than “charging only”?

Whenever my device (Nexus 5, Android 6.0) connects via USB, the USB config always defaults to "charging only".
This is a big problem since whenever I'd like to listen mp3 connecting nexus 5 to my car stereo I have to manually set the mode to MTP.
The big annoyance is that whenever I turn off the car engine and then restart it the nexus 5 USB connection go back to "charging only" !!!
I already tried debugging mode:
Develop settings -> USB Configuration is set to MTP.

Any chance to set USB default as MTP?

Thanks