What folder on android does show as a media device? When I connect my phone to my car via USB it says connected as media device.  Then when I go to browse with my car nothing is in the folder.

I use Google play music and know the folder it caches my downloaded playlists.  I want to make a symbolic link to that folder from the folder my car is seeing.  Does anyone know what folder is mounted by default?  Or maybe a better way to play music in my car?

HTC One M8 Converted Google PLay Edition.  2014 Mini Countryman w/out MINI Connected.
First, that depends on the protocol. AFAIK, when using PTP, only the DCIM folder (with your photos) is shown, while in MTP mode, it should be your entire SD card(s).

Second, I doubt whether a symlink would work here – at least not when it points to the internal storage (below /data, where Play Music usually puts its stuff).