Q: ios 8 installed and now bluetooth in car not working

Basic trouble shooting steps are Restart then Restore in that order: Turn your iOS device off and on (restart) and reset

Some more BT troubleshooting information: iOS: Troubleshooting Bluetooth connections

You didn't say what kind of car you have but apparently Fiat has to issue an update to fix bluetooth iOS 8 compatibility issues for their cars. Perhaps your car is the same.

I"m having a similar issue. If I manually dial (I have a 2008 Toyota Highlander) the bluetooth connection works fine. If I use Siri to call, it drops the connection shortly after it begins dialing. So in my case it appears less of a bluetooth issue than an odd Siri calling issue.