Rectangular LED lights are commonly used as front/turn/clearance lights or stop/turn/tail lights. They can be mounted in either a rubber grommet or a metal safety flange. Many of these lights don't require any additional mounting brackets - they simply mount to the truck in a surface mount or flush mount style. It's not uncommon for a truck to have a few of these on their mudflap hangers or on their trailer. They are less common than the 4" round styles, but they are still in widespread use.