Listener Claire called in last week seeking help to keep from leaving her headlights on all the time. Click and Clack (“Those two knuckle heads”) suggested paying an auto electrician large coin to re-wire her VW to have the lights cut off with the ignition. That is like using a jack-hammer to loosen a small screw 
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Claire needs to just buy a “headlight on reminder” buzzer / beeper. Like these:
http://www…_10101.jcw
http://www.liteminder.com/left.html
http://www…ical01.php
You can also make your own with a 12v piezo buzzer from Radio Shack along with a couple of fuse taps - also available at Radio Shack. You connect the negative (black) wire of the 12v piezo buzzer to the side of a fuse that gets +12v when the key is on - like the radio fuse. You connect the positive (red) lead of the buzzer to the side of a fuse that gets +12v when the lights are on - like the headlight fuse. You will have to test to see which side of each fuse to use, but that is easy. Then if you have the key on and the lights off - you have -12v on the buzzer and no noise. Light on + key on = 0v across the buzzer and no noise. Lights on + key off you have +12v (more or less) across the buzzer and you get noise. I have done this add on to many cars for myself, my wife, and friends. I usually get a really loud buzzer so they will not ignore it. Just Velcro or double sticky tape the buzzer up under the dash once you have the wiring done. Or buy one of the ready made kits.
Keep up the good work Click and Clack, but it is MUCH easier and simpler to make the car beep to remind you to turn off the lights than it is to make the light go off (or to make Claire able to remember to shut them off).
This is no help if she looses her keys or if she walks away from the car each time saying to herself “What is the loud noise?” .
Where Can you get a piezo speaker, I had to buy a few for our schoendstats (metal locators), but the dealer prices are outrageous, and my mind went into hypermeld after imagining your schematic, positive to positive, Wouldn’t the speaker need a ground? I am just wondering why the thing fires? And if the fuse is good what difference does it make which side you tap? I am not discounting your successful applications but having trouble understanding it. I was thinking a more simplistic thing like when you turn the lights on you slip a key ring from the light control to your finger, then when you turn them off put back the key ring. I sure do love those headlight minders!
search for piezo buzzer .
A piezo buzzer is not a speaker, it produces a tone (or more than one) when a DC voltage is applied across its terminals in the correct manner. The ones I used have a working voltage of say 5v to 20v DV. If you apply the voltage correctly (+12v to the red lead, ground to the black lead) it sounds off. If you connect it backwards you get nothing. If you connected it with both leads to +12v or ground you would get nothing. It only sounds when + DC voltage is applied to the red lead and a LESSER voltage or ground is applied to the black lead.
It you connect the red lead to the “parking light” fuse, for example it would have no voltage on it unless the lights were on. When the lights were on, it would have +12v (or whatever the battery / alternator voltage) on the red lead.
Now if you connect the black lead to ground it will sound. If you connect the black lead to a lesser positive voltage in the car it will sound as well (but not as loud). So you connect this lead to a fuse that has no voltage on it when the key is off - like the radio fuse, rear window defroster fuse, wiper fuse, etc. When the key is on, this connection has +12v on it and the buzzer can’t sound. Turn the key off and leave the lights on, and the red lead has +12v on it and the black lead has “something less” so the buzzer sounds. There is enough difference of potential and current flow to ground to make the buzzer sound when the key is off to make this work - in every car I have tried - and is the idea behind the “commercial” headlight minders I linked to.
I have seen some cars that have “always hot” fuses which mean you would have to tap into the system in another location that the fuse box, but most cars are wired so that the fuse has +12v when the devices connected to it should be on / could be turned on and nothing when they should not / can’t be on.
Unless you want to wire a fuse inline with the piezo buzzer, you would want to wire it in to the side of the fuse that has NO VOLTAGE when the fuse is pulled that way the car’s fuse is protecting the new buzzer. Other than that, it should work wired to either size of either fuse in most cars.
To me it sounded like a speaker, and I appreciate your enlightening in depth discussion. I learned something new today, thank you. Should be schoenstadt metal locator in previous post.
I think she should take her VW to dealer where they could change setting of headlights in car computer so that warning buzzer is turned on.
Claire said she has a 1991 Golf. I don’t think there is any such setting in that year and model car.