So, before starting, I'll let you know what I used.
- A car with a USB entrance (2011 Honda City, in my case)
- A phone (I am currently using a gt-i9300 with temasek's CM10.1, may or may not work with others)
- SGS3 Easy UMS
- (Optional, may be necessary): USB to microUSB cable
- (Optional, may or may not work otherwise) A microSD card, also known as an external memory card.
So, to summarize, I'm using a 64GB (FAT32 formatted) microSD card, with my GT-i9300 with an AOSP rom and a Honda City.
I am also storing ALL my music on the external SD card. I am unaware if this works on the internal SD card, since I have NO music files on there and I haven't bothered to try it out.
Steps I have taken to get it working:
1- Connect to the car's USB port.
2- Run SGS3 Easy UMS
3- Give root permissions and select UMS mode (will take roughly 5 seconds to activate).
4- That's it. Don't forget to set your device to MTP if necessary (if connecting to computer or whatever, and it's not recognized in UMS mode).
Alternatively, in my case it doesn't work anymore after I replug the USB cable. Using the "Mount card" or "MTP" options does nothing for me, and there are 2 ways to get it back working:
2- Go to storage -> Unmount card -> Mount card back again -> select UMS mode again.
Images attached as proof.