Depending on your car audio's system, you may have to set it for Bluetooth AUX input in order to hear audio from your iPhone's navigation. If you're listening to AM/FM, CD, or satellite radio then your AUX input is not active, which means that the Bluetooth navigation prompts from your iPhone may not override what you're listening to. Perhaps that explains the intermittent nature of what you're experiencing (i.e. when you're listing to Bluetooth audio you can hear navigation, but not when listening to other audio sources).
Some car audio can let your iPhone override whatever you're listening to in order to hear navigation prompts. Such audio systems allow the navigation prompts to come in over the "phone call" Bluetooth profile (HFP, Hands Free Profile). The iPhone sends navigation audio over HFP (and A2DP), but your car may not support HFP or there could be a compatibility issue between your car audio system and how the iPhone handles Bluetooth. Make sure that your iPhone is up to date, and your car audio (sometimes there is a software update). You may also have to delete the iPhone from the car audio and re-add it.
Question: why does the audio for the Mapping apps not always work?