Originally Posted by GuyInDogSuit
Is there any way to get UMS working?
Well kind of.. sort of.. maybe.
You should be able to get UMS running, but I won't promise that you'll find it easy to do. Samsung doesn't really support the older UMS mode. Which is a shame.. while UMS is an older and more simplistic protocol than MTP it simply works with most devices. Still most of the blame lies with your stereo vendor which claims to support Android while eschewing MTP support, which is what most Android devices use..
Anyway, as you have discovered, Samsung basically has MTP and PTP. End users can pull down the notifications bar after connecting an USB cable to a device and click on the "Connected as.. " notification to switch between the two modes. Or you should be able to pull up the full USB options menu with the dialer (keypad) code *#0808#
As to getting UMS to work.. you can certainly do it if you are rooted and using a capable kernel.. i.e. a kernel that supports UMS, ntfs/hfs/hfs+, etc. If you aren't rooted, you will have to experiment with different third party apps (and possibly update your stock firmware) until you find a combination that works. Also keep in mind that UMS is only likely to give your stereo access to the SD card in your phone.. not the phone's internal memory.
Some apps to try include:
Easy UMS - http://forum.xda-developers.com/galaxy-s3/themes-apps/app-sgs3-easy-ums-mount-external-t1711009
SG USB Mass Storage Enabler - http://forum.xda-developers.com/showthread.php?t=2297888
USB Mass Storage - Google Play store