Hope someone can advise.

iPhone 5 pairs with vehicle BT system as it should, but when in 'sat nav' mode (the apple app for maps that replaced google maps) something strange happens.

The verbal instructions from the phone are muted, but the vehicle system remains silent. There are no verbal instructions. If you disable BT on the phone, then the handset starts giving verbal instructions again. I've messed around with settings but I can't get the verbal instructions over the car speakers.

Any ideas?