There are three pieces. One is the outside collar that is inserted from the outside. The second is the sensor. The third is a collar that holds the sensor and is installed from the inside of the bumper. Now, the inside collar fits only a specific way and clips into the outside. It is "keyed" in a way that it only fits one way properly. It is too tight to uninstall or reinstall without dropping the bumper. At least I couldn't do it.