This Blind Spot Sensor Kit with LED & Audio Warning Sound is a safety must have.  

When changing lanes blind spots create an incredible hazard and are a leading cause of accidents.  This system includes 2 sensors that are designed to mount the left and right side of the rear bumper, these sensors look identical to the back-up sensors but are designed for side detection. Also, included are 2 LED indicators that can be mounted on the left and right of your dash or instrument cluster to visually indicate vehicles or other obstructions.