The trailer brake signals go through the CANBUS system in the Sprinter. There is no voltage supplied until a resistance is measured on the line. So, you can't check at the trailer connector with a multi-meter or a LED check light but a good old light bulb test light will work fine. Just another weird Mercedes quirk. BTW, you can't really use the 12V line for much of anything either.
Wire your car up, add the diodes as needed, and plug into the Navion. It should work fine. I used a Tow Daddy system (bought out by Hopkins) to wire up my Honda Fit and it works fine with the Mercedes CANBUS.
Roger & Mary
2017 Winnebago Navion 24V (Sold)
2014 Tiffin Phaeton 36GH (Sold)