It sounds like you have the automatic dimming mirror with HomeLink. There are three programmable remote control buttons for HL. There are two other buttons marked "I" and "0", which are the internationally recognized symbols for "on" and "off", respectively; these enable or disable the auto-dimming function.
... which is pathetically useless in my opinion. A manual dimmer switch would have been better because it seems that the rear spoiler blocks the following headlight from the detector and results in no dimming effect for you as the driver. This has been debated at length elsewhere on PC.
