On your iPhone go to Settings  and the General item under Settings. Go down to Bluetooth and turn it on. The iPhone then becomes discoverable for a period of time. The iPhone should see the Hands Free system of the car and then display a 4 digit form (and keyboard) to enter the pairing passkey given by the car.

Why wont my phone connect to my car USB?

Not all USB cables will work with all cars. If youre having trouble connecting to Android Auto try using a high-quality USB cable. Make sure your cable has the USB icon . If Android Auto used to work properly and no longer does, replacing your USB cable will likely fix this.