It works about 10% of the time. Not sure why this has been a perpetual Gap for Tesla . Every other car, even economy cars, have rock solid blind spot detection using the same hardware.
There are a lot of things wrong with the blindspot monitoring in the Tesla.
The indication/display is in the Instrument cluster, but most people will be looking at their mirror.
The indication often lags quite a bit, and this is likely due to latency in the display electronics.
The ultrasonic sensors seem to miss things. The blindspot monitoring systems I like tend to be radar based, and not ultrasonic based.
For those of you with AP2/AP2.5 I do expect blindspot monitoring to improve immensely because they'll eventually use information from the rear facing side cameras.