I'm also having the same problem as you noodlemctwoodle on a Nexus 6P with 7.1 developer preview.
I've un-enrolled from the beta program this morning, waiting for the ota to come down, as soon as it does will re-test and let you know if it's worki...
bugger, thought it was quite promising when everything started to connect itself :)
I've been building my own app to do simple notification mirroring over a wifi tether, but this would have been much easier haha.
Also cloned and starting to build it as am having issues with my Joying head unit showing black screen after connecting (Test shows a black screen also).
looking forward to getting this running.
Oh I see. You can use mtc-keys to remap those buttons to launch the MyMusic and MyRadio activities.
cheers, I'll give that a try next time I get in the car :)