Thanks, Nate.
I found instructions for doing the diagnostic mode
http://mr-fix.info/hidden-menu-bmw-navigation-mk3-mk4/
So I’ll check that out this weekend and see if I can narrow down what’s going on.
RealOEM wasn’t much help finding the antenna - it shows the part and the cable, but no context for location. I’m hoping it doesn’t involve pulling the headliner...