I to went through a painful ordeal with my car's BT. The 2014.5 Mazda CX5 and Nexus 4 did not like each other. Every 3-4 days it would forget the phone or visa versa.
My G3 on the other hand has been paired with the car for 3 weeks now, even text msgs come through.
Unfortunately, it comes down to both phone and car playing nice. Maybe the car company may have a software update for the audio system? Have you fully reset the cars BT memory to see if that helps?