Collectively, the userbases for Android M and Android Auto have to represent only a tiny fraction of the total users of Android. But those people are also the most dedicated Android fans, so any developer hoping to get attention from power users would do well to consider them. Case in point: BeyondPod, the go-to podcast manager. This weekend the developers posted new betas with support for Android M and Android Auto on their official forums.
Notice the plural there: you can't get support for both M and Auto in the same app, at least not yet. The 4.1 branch currently supports Android Auto head units, and the 4.2 branch supports the versions of the Android M Developer Preview that have been released so far. That's a bit of a bummer for the even tinier sliver of Android users that need both, but the respective updates should come to the public release version before too long. The 4.1 branch also includes an option to automatically delete all played podcast episodes when it updates feeds.
To get access to the betas, you can join the official Google Group, download the 4.1.44 build from the website itself, or download either one from APK Mirror. Note: According to the chgangelog, you MUST download the beta from the Play Store via Google Groups in order for Android Auto to recognize the app - Android Auto doesn't play nice with sideloaded APKs.