Just wanted to write this up as a PSA for owners of 2014 or newer Sprinters with the Blind Spot Assist feature (the triangles that light up in the side mirrors).
After weeks of troubleshooting, I have confirmed that the factory trailer hitch wiring harness can cause problems with the blind spot system. Basically if anything is plugged into the 7-way connector, the van computer disables blind spot monitoring since it has no way to judge the length of your trailer.
The problem is this also happens if the connector THINKS something is plugged in. In my case, the cables got pinched by the protective bracket (pictured), possibly from bumping into a brick wall while parking. This caused just enough of an electrical short that the van thought a trailer was connected. While driving on the highway, I would get an error message "Blind Spot Assist Deactivated" followed by "Blind Spot Assist Activated" with no rhyme or reason. The dash doesn't say anything about a trailer being connected.
I only checked the hitch wiring after finding this post
on the Sprinter Source forum. In that owner's case, the cables weren't pinched but the connector was full of mud that caused similar shorts. A Roadtrek owner also reported the quirk
, traced to a light that was installed for the spare tire carrier.
Fortunately a Sprinter dealer fixed my pinched cables under warranty and the system is functioning fine. Electrical tape probably would have accomplished the same thing for an out-of-warranty repair.