We believe the Tailgate Mirror fits on all Ram trucks up to 2018. It does not fit the 2019 Ram, The coming bracket system will work on the latest Ram trucks.
Tailgate Mirror installed on a 2017 Ram 2500
This is with the tailgate up for easier viewing (also easier to install)
Below shows with and without the Tailgate Mirror on an F150. It will look very similar on the Ram.