Once you move to iOS 6, you can't downgrade. iPhone 5 was designed for iOS 6 - so even if you could downgrade, you can't :(
The work around for now seems to be to use iPhone to change audio source from iPhone, back to Mazda and then audio routes via the car as expected.
If you like Google...
Intermittant: iPhone 5 / iOS 6 issues with Bluetooth
There's another thread of iPhone 5 here, but it contains other information about streaming music, so I thought I'd start a new one.
When using an iPhone 5 and BT Audio - most of the time, incoming calls do not have any audio available to the...