To dream of a car alarm represents feelings about warnings or alerting signals in life about interference with decision-making. Shock or surprise that someone is trying to cease control of decision-making or interfere with your ability to lead your own life. Fearing unwanted control over your decisions.
Example: A woman dreamed of seeing someone jump off a building and smash through the back window of a car setting off the car alarm. In waking life she had decided to take a big chance and move away from her cheating husband which ended up making him respond by trying to stop her. The car alarm may have reflected her feelings about her husband being tipped off about her attempts to leave the home which would interfere with his ability to make decisions as the head of the home.