Reproduced end cap for VW Vanagon Multivan mirrors. In transparent plastic (Frosted Detail) these can be used to kit up your mirror with side lights and blinker lights for better visibility.
The small tower in the middle is used to secure your electronics in place. LED's can be fixated around the tower with hot glue. If you use a circuit board make sure there is a hole in the middle and the tower can be used as a fastener by melting it down with a soldering iron or used as an anchor for hot glue.
The original cap has a plastic spring clip to lock the cap to the end of the mirror. This cap has a hole threaded for an M4 stop screw for a secure mount.
You have to order two of these to make a pair.
Installation instructions can be found here