Most likely signal, (1290 is not a strong signal around the UofM campus), and I'm not convinced that the programming is interesting to Liberals.
It's been my impression that although Air America carries the banner of Liberalism, it just doesn't attract the typical Liberal/Progressive listener. Most of the people I know in the Ann Arbor, Ypsi, Detroit area that I could characterized as Liberals, listen to music radio, NPR, and other venues.