Error connecting S4 to card radio USB - help?

[h20polo1]

I can successfully connect iPhones, iPods, USB memory sticks & Blackberries to my card radio USB input (2012 Toyota). However, connecting the S4 results in an "error" on the radio display. I have contacted the car dealer and they have no clue! Does anyone know of a phone "tweak"/setting to get this device recognised by my radio?
PS: The device does charge over the USB and the phone does report that it is connected as a media device on the phone but it does not playback through the radio.

 

[h20polo1]

Update - a bit of searching suggests this is likely a result of the fact that the S4 has USB modes MTP and PTP - importantly not UMS (USB Mass Storage) mode! Apparently I am not able to get this mode: a) without rooting the device, & b) to make the internal storage available. If I root the device, the best I can hope for is getting a SD card recognised.