Android USB audio output NOT working in my car

[USB_addicted]

Hi People!

Here's the story.

I have a Fiat 500 with a 2015 Fiat original USB radio. It does not support bluetooth audio streaming. Only AUX through 3.5mm jack or USB AUX. Or well, I should buy one of those bluetooth aux dongles.

If I plug an iPhone via USB, I can listen to Spotify though my car hifi with no hassle, using a normal micro USB (+thunderbolt adapter) to USB cable.

I have tried to do the same with 3 android phones:

- Samsung Galaxy S6 NOUGAT stock 7.0 rooted

- Samsung Galaxy S7 OREO stock 8.0 rooted

-Samsung Galaxy S3 NEO Nougat AOSP 7.1.2 rooted

No chances to tell the phone to send the audio through the mini USB port. I am using the same cable that I used for the iPhone, without the thunderbolt adapter, obviously.

My phones are all rooted and I have tried to modify the usb functionality inside the Android developer options. I have also tried an app (Audio Router 2.2 updated on 2018-01-22) that sould force the usb audio routing, but with no luck.

Is there anything I am missing? I have tried android auto but it did not let me add Spotify to the dashboard.