I have a 2014 Subaru Forester.
Previously, I was using an iPhone 4S on iOS 7 and every time I started the car, the phone would automatically connect to Bluetooth. Worked great.
I switched to an iPhone 5S on iOS 8 and that stopped working.
I tried deleting and re-pairing through the radio about a dozen times. Still didn’t work.
Finally, I remembered the audio pairing option available through the steering wheel control. Turns out, I think I had maxed out the number of connected devices there. Apparently, the radio and audio settings don’t interact with each other.
I talked through the BT setup options, deleted all the devices I had installed there and re-paired my iPhone.