Connecting Echo Auto to exiting Bluetooth car devices, like Pure Highway
If you've previously added smart connectivity to your car - like a Pure Highway digital receiver for example - then you can continue to use those devices setting up the Echo Auto as though you're connecting to a car that accepts Bluetooth streaming. As long as your phone is connected to that other device too, you should have no problem.
However you really need to consider why you might want to do this. In many cases the Echo Auto's functions will duplicate some of the common added options - digital music and streaming. There's a cost to that, of course, in that you'll have to have a data plan to support music steaming, whereas DAB radio is effectively free.
There's also more potential for failure the more devices you start to daisy chain. Each connection is a point of failure - your car, your phone, Echo Auto, whatever else you might have. We connected the Echo Auto to a Pure Highway 400 and found it worked with no problems, but you're really not then using the Pure device for anything other than its Bluetooth connection.
https://www.pocket-lint.com/cars/news/amazon/152634-how-to-setup-your-amazon-echo-auto/