The boot latch on the Ford Focus works in a similar fashion to the door handles, but without a latch. If the latch on the boot is having problems, the most likely result is that the boot won't stay open. In this case, you need to fix the boot by replacing the latch. If you are fixing and replacing the latch, you need to transfer the old lock cylinder to it or install a new cylinder with it.
Disconnect the car's negative battery cable, especially if the car is equipped with power locks.
Open the boot.
Undo and remove the plastic screws from the trunk's liner trim, unplug the wiring plug from the latch assembly and unclip the rubber grommet that surrounds the lock cylinder inside the boot lid.
Unscrew the mounting screws for the boot latch using a Torx wrench, withdraw the latch with the lock cylinder and disengage the cable end of the latch.
File down the housing of the lock cylinder until you can see the spring and plunger. Remove the spring and plunger and then turn and remove the cylinder from the housing.
Cut a thread in the replacement latch's lock cylinder housing using an M6 tap. Cut it in the spot where the plunger was removed in the old cylinder housing.
Install the lock cylinder into the replacement latch's housing and turn the cylinder to locate it into place. Push the plunger into its hole and secure it with an M6 by 6.5 millimetre retaining screw with Loctite sealer applied to the threads.
Insert the latch assembly into the seating in the boot lid, connecting the cable end, and secure it with the mounting screws.
Clip the rubber grommet in place on the lock cylinder, connect the wiring plug to the latch and screw the boot liner in place.
Insert the key into the lock cylinder and ensure that it turns freely and the latch properly functions, then close the boot lid.
Reconnect the battery cable.