TLDR: I tested it with Amazon Music, making announcements etc, and it works just like the home units with decent sound through the car speakers.
I know this is a late reply Peter, but I recently installed an Echo Auto that I picked up for £25 on Black Friday. My car is an IS250 - without nav, and I found that my phone usually connected for calls only, with no streaming. The non-nav bluetooth setup menus are awkward, so I needed a workaround. To avoid trailing wires I connected the Echo via the Aux socket in the centre console and I leave it in there connected to my phone.
My phone has very long battery life so I mount it horizontally on the dash as a satnav display, without trailing cables, and the 'Android Auto for Phone Screens' app handles voice commands for Amazon Music, Waze etc. If I ever want to use the Echo I can slide back the centre console lid and it picks up voice commands.
If my regular bluetooth connection was more reliable I would have simply used Android Auto but the Echo bridges the gap for me.