Inside each car model folder you have a "Skins" subfolder ... that's where they go.
Each car model skin in its own Cars/CARMODEL/Skins" folder
Sometimes, the zip file has the whole path ( beginning by the Content one ) and may be unzipped in the AC root folder ... sometimes not.
So for a manual install, open first the zip file in order to check how the zip was built before unzipping it ( maybe in the wrong place ).