Check this out at Amazon.com
Side Skirt Retainer Clip with Sealer, fits for Ford (Pack of 20)
Amazon.com: Side Skirt Retainer Clip with Sealer, fits for Ford (Pack of 20): Automotive
The company I used to get all of my parts from in UK is out of business now - but I believe I had something like those - but in a Ford supplied kit. You'll need 3 or 4 of those each side to affix the skirt to the UNDERSIDE of the car. If you look unde the side of the Fiesta just past the pinch weld you'll see this row of black rubber plugs. You don't need to remove all of them - maybe every other one - but that is where you pop those round white retainers into.
There is a little self tapping screw that holds the skirt in the back - inside the rear wheel well. And the end cap to hold the skirt in place in the front wheel well.
The actual STs have.... 8-10 little holes in the rocker panel to hold little plastic rivets that attach behind the skirt to keep it tight against the body. I just ran a strip of double sided 3M trim tape along the top edge of the skirt and pressed it tight below the doors. I'm not worried about it ever coming off, heh.
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