I'm still wrong... Nothing is going to get to the 'lock doors' light through R12. It must be blocking what's coming from the bias on Q11. This wouldn't be enough to send a false signal to the light, but the presence of the bulb would throw the bias off without the diode. So.... Replace with the following: "The purpose of diode D5 is to stop the 'lock doors' warning light from pulling bias away from transistor Q11. There is no equivalent diode D5 on the lock doors circuit because there is no equivalent 'doors are already locked' dash warning light to interfere with transistor Q1. The function of diode D13 is to allow the transistor to be switched on without the door unlock switch interfering with the transistor's bias." I'm sure there is another point or two which I have wrong, so I'm open to criticism. Walt Tampa, FL