Tonight a message showed up saying that my left brake light was not working but on inspection it is working fine. Does this mean that a sensor is going bad? Can i make it so the message does not come on every time at startup?

 

are you sure it works for both brake and marker?

 

This same thing happend to me this summer. The tail light was functioing fine and the warning message appeared. I verified that the light was working and then hit the clear button to make the message go away. about two weeks later the light stopped working and the message appeared again...this time with additional warnings (directional light failure, brake light failure, etc.) all three lights in the cluster had failed. The newer saabs can sense when a light is about to fail. I think that's what is happening to you.