Is Power Amp installed from Google Play or side loaded? Android Auto will inexplicably refuse to launch side loaded apps half of the time. If it is launching but no audio is coming out, this isn't the solution for you
To get around that I download my app (in my case, CoMaps) via Obtainium + Shizuku with the option "Set Google Play as the installation source"
Since Poweramp isn't FOSS, you'll have to download their provided apk & move it to /data/local/tmp with adb
Then you can run pm install -i "com.android.vending" /data/local/tmp/Poweramp-build-999-uni.apk
This will overwrite the installed app's installation source with Google Play and let Android Auto work with it
