Pair iPhone with car and redirect navigation (instructions) to the car speakers?

Hi,

My neighbor asked me today if I could help him pair his iPhone with his old VW Passat. No problem, I thought, and paired the phone with the car via Bluetooth. Playing music works without any problems. That means I set the radio in the Passat to "Media" and select Bluetooth as the source. Music plays from the phone.

However, when he wants to navigate now, the voice announcement comes neither from the mobile phone nor from the car speaker.

What am I doing wrong?