How do I stop automatic Bluetooth streaming?
Turn off bluetooth on the phone.
Unpair the phone from the truck.

When you pair your device, the truck is seen as a remote control in addition to all the other functions so if the truck wants to play, the phone is right to start playing. There is no switch to disable one remote some of the time in iOS or to edit the capabilities of a paired device.

You might need to go to the ford dealer and see if they have a firmware update for your truck :-) Seriously - they either need to disable or let you better control the situation where the "remote" asks the phone to play music.