actually - for MBs with the radio prep option, i.e. no factory installed radio, you want a simple harness -
Metra 70-1784 Radio Wiring Harness
https://www.amazon.com/gp/product/B0002BEQKM/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
This is what I used - was easy to move the pins to the proper position to match the factory harness for this option - which is referred to in the diagram on this thread. if I get some time, I'll post up a wire mapping I think I have somewhere...
Metra 70-1784 Radio Wiring Harness
https://www.amazon.com/gp/product/B0002BEQKM/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
This is what I used - was easy to move the pins to the proper position to match the factory harness for this option - which is referred to in the diagram on this thread. if I get some time, I'll post up a wire mapping I think I have somewhere...
I think the problem you encountered is the 1784-NAV harness is not the correct p/n for a 2014 and on Sprinter.
The correct harness is XSVI-9005-NAV I spoke to Metra today and confirmed for anyone who wants to know.
http://metraonline.com/part/XSVI-9005-NAV