Just a couple of days when we sent word which the Galaxy S6 — plus some other popular phones are not working properly with Android Auto, a fix has arrived. It's not a large shock which the latest version of Google Play Services (which manages a range of Google apps with your phone) rolling out now enables Android Auto on these popular devices, and we're willing to say that indeed the GS6 is working.
So position the GS6 from the good column, together with the HTC One M8 and M9, the Moto X, Nexus phones among others.
The version numbering in the Play Services app (which really mustn't be user-facing inside the first place) varies by device, so there is not a whole lot you can use here — just wait for update that's pushing now, once it arrives you will end up all good for just a friendly in-car experience on the shiny new phone. A good number of folks inside our forums are locating what's working currently.