Thus far I have not had any issues with our cars not having a blind spot warning system. Having said that I would like to have the option of adding the feature for peace of mind. There are a number of aftermarket units out there. I remember reading about one system that ties into the car’s system. Unfortunately the built in sensors in the bumper are short distance units primarily used for parking and probably will not work for blind spot monitoring. One system i saw uses units the fit inside the bumper so that you do not need to drill into your bumper. The sensors are then hardwired to the warning lites that can be placed near the front windshield pillars. You then need to connect to the obd2 unit or input for the gps system so that the car knows when it is moving. Has anyone looked?