what is the red botton in the trunk for?

This mysterious red button you inquire about is a safety device built into Fords called an interia switch. It is designed to disable the electric fuel pump in the event of a crash. It disrupts the electrical current to the fuel pump if it is impacted at a predeterermined calibrated force. The button resets it if it is activated. Typically, it is never a concern, but it is a good place to test for fuel pump power.