That one still stumps me. Reagan defied trends in other Rust Belt states, winning working-class Macomb and Saginaw, while also flipping Washtenaw- home to the University of Michigan- against Mondale when he had been making inroads in "liberal elite" areas elsewhere. I always assumed it was a reaction to crime and something having to do with the state's notoriously bad race relations (1967 Detroit Rebellion fresh in memory and all that). I had wondered if Gerald Ford campaigned for Reagan that year but it doesn't look like it. Or maybe the United Auto Workers had some kind of beef with Mondale?
Macomb County used to be relatively high income compared to the United States average. A Lawrence County PA it was not. But industries in Michigan have declined a lot since then.
https://www.nytimes.com/1972/09/19/archives/50-richest-counties-are-in-suburbs.html