Normally, it is a matter of, within your car's BT settings on your BB, ensuring that the car profile is Trusted as well as set for auto-reconnecting (along with activating any other settings that appear to influence this situation). If you car BT system has any such settings, you'd need to also configure those in that manner.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.