iPhone 6s Plus won't Connect to Pioneer DEH-6600BT Car Radio

Last weekend I got myself a new iPhone 6s Plus in anticipation of using its GPS navigation capabilities on a road trip from California to Texas. (I got it for everything else too, but had the road trip on my mind when I bought it.)

However, every time I have tried pairing my phone to the car radio (Pioneer DEH-6600BT) it doesn't work. While the radio picks up the pairing, the iPhone recognizes the radio as a pair-able device, and the radio sends over the security code, I always get a "Pairing Unsuccessful" message (see attached screenshot).

When I try connecting the iPhone automatically to the radio, using the phone's USB cable, the radio displays "FORMAT READ" on the prompt but no volume or access comes out of the system. At times, the radio even displays a Bluetooth passcode, like when it's trying to connect via bluetooth, to the device.

I've tried the various methods for other radio systems and phones mentioned on the various threads online, but none of them have worked so far. I'm hoping someone has a solution to this problem as this is becoming an inconvenience and I'd rather avoid having to replace the car radio (again) if I can avoid it.

Thanks!