The Michigan State Police website has a section about "proper conduct during encounters with police". Since they don't actually cite the statute, I'm naturally suspicious about the information being skewed. However, they do expressly mention being "stopped by a police officer (traffic stop or otherwise)."
I'm betting this is a gray area--undecided by case law/court opinion.
I vaguely recall that federal case law considers the passengers "stopped"/seized for the purposes of the 4th Amendment. I'll bet a jerk cop or prosecutor would argue that since you were a passenger, you were "stopped", and thus compelled by law to disclose the gun. All it would take is an anti-gun or pro-cop trial judge to agree with their argument.