For adequate support, you can mount exterior light fixtures to a solid block or base, which you usually install before or during the siding process. However, when retrofitting lights on vinyl or aluminum siding or fixing a sagging fixture it's usually easier to buy special blocks that fit right over the siding (available at home centers). The mounting blocks are paintable and available in different sizes and profiles to fit most types of flat and lap vinyl siding styles. You don't have to cut into the siding to mount the block. However, light fixtures must be secured to an electrical box. The block we show here has a built-in electrical box, so you don't need the one in the wall. If you use a block without an electrical box, shift the existing electrical box out so you can mount the fixture to it.
To install the kit, insert the cable connector in an opening in the center of the mounting block, then feed in the cable(s) from the back. Set the block into place as shown and screw it to 1/2-in. (minimum) wood sheathing. Or mount it to a stud. Connect the wires, mount the fixture and caulk around the block so it's watertight.