How to install a "garage door open" indicator
http://www.instructables.com/id/How-to-install-a-quotgarage-door-openquot-indi/

How to install a "garage door open" indicator Ever left your garage door open overnight? In addition to being a terrible energy waster, all the tools, bikes, and other stuff are available to thieves - who might even enter your home via the door from your garage to your living quarters. Install a simple indicator which will alert you if you've left the garage door open. Total cost is less than $30-$40, and you can determine exactly where the indicator will be located.