How do I switch from my Phonak hearing aids to the car speaker?

Because your hearing aids are connected to your phone, they will take priority over the connection of your phone to your car. The car connection is still there, you simply have to change the setting. There is no need to change or disable your hearing aids.

On your phone, under Bluetooth, you will see all the devices that your phone is connected to. When in your car, go to this menu and click on the car Bluetooth. This will then switch the connectivity from the hearing aids to the car. When you exit your car and walk away, the phone will then connect back to your hearing aids.

You can also change the priority order of your phone. This will involve pairing the devices in the order in which you want connection. Under Bluetooth settings add (or remove and re-add) the devices to change the priority order. The devices should be listed in priority, and attempt to connect in the order listed. If a device with higher priority is available during a periodic scan, it should attempt to drop the connection to the current device and reconnect with the higher-priority device.
If you want your car speakers to have priority, then they should be paired after the hearing aids are paired. When you connect to the car speakers, the hearing aid connection will be paused until you exit and walk away from the car.