Volvo Repair: Gas Cap Cover, plastic hinge, plastic pins

Question
I have the parts, but can't figure how to install the black u-shaped piece ONTO the fender.  This is the unit which holds the spring and cover to the gas tank fill port.  Are there suppose to be screws placed from the inside of the trunk?

Also, does the spring and cover have any additional hardware, can't fiqure what actually holds the whole thing together?/

Thanks , David

Answer
it has 4 plastic pins you need to punch in when it is on the body.put the hinge on the gass door firts before putting it on the car.then place it on the car and use a hammer and punch to push the pins in.then the spring fits into 2 tabs on the plastic hinge.