Because of the older year of that vehicle, The only thing I can think of is that the vehicle has a different Bluetooth capability setting that's pairing with your Dad's iPhone, which it recognizes. From my experience, It's not specifically your iPhone 6s that's not recognizing the Bluetooth, it's actually the vehicle that's not recognizing your iPhone 6s
Does your iPhone show any type of signal that it recognizes the vehicles Bluetooth and you jus cannot connect? Or is it is it you just can't see it at all in your iPhone.