if holes are not its just a drill bit same size as the plastic clips,, failing that its poss to bond plastics on to any car ie stuff called tiger seal will do it link below
Tiger seal is a one component ready to use polyurethane sealant and adhesive that bonds to most materials including steel, aluminium and most plastics. Ideal for sealing and bonding body panels, trims and seams.
www.ebay.co.uk
i've installed many skirts and spoilers etc over the years on all kinds of stuff that never had them in first place, once it goes hard its never coming off,, even played a joke on a mates old van once ie sealed his back doors up for a laugh,, ho thats how we found out how good it is lol,