I admit it. I’ve left my lights on more than once and run the battery down. This afternoon I came up with a cheap, easy solution using parts from my junk pile. All you need is a normally open relay, an old seat belt buzzer, a couple of those wire splice things and some short scraps of wire. Installation takes all of 20 minutes. If you remove the glove box liner you have ready access to all of the necessary wiring- at least on xr7’s. Splice a wire into one of the bulbs on the oil pressure gauge and connect it to a terminal on the coil side of the relay. Run the other coil side terminal to ground. The hinge screws make for a convenient ground. Splice another wire into the pos. wire of the courtesy light that’s also conveniently right there. Connect that wire to the common terminal of the switched side of the relay. Finally, connect the buzzer between the other switched side relay terminal and the ground. Tuck everything up out of the way and put the glove box liner back in. Now, turn on the lights and open a door- your buzzer will sound until you either close the door, turn off the headlights or both. CAVEAT: of course this means you won’t be able to use your courtesy lights while driving at night without the buzzer sounding. But you’ve still got the map and sail panel lights to light up the cabin if needed.
I’ve used one of these kits in ebay link before. If you wire them into the instrument light circuit like you indicated, you can actually park with the parking lights on by dimming the instrument lights
although this one is a bit expensive, I’ve seen them in electronics stores for $10.
For non XR-7 owners, here is how you can build one and hide it anywhere under the dash. Of course, routing wires carefully and making sure they don’t chafe, and adding fuses as you see fit.
you can build one using these two parts:
http://www.alliedelec.com/search/productdetail.aspx?SKU=70175709
and
http://www.alliedelec.com/search/productdetail.aspx?SKU=70186649
You would wire terminal 85 to a 12V source that is hot only when the key is on, and wire terminal 86 to a ground. These are the coil leads for the relay.
Then wire terminal 30 to a wire that is hot when the headlights are on and terminal 87a to one lead of the buzzer. Wire the other lead of the buzzer to ground.
That way when the key is off and the headlights are on, the buzzer gets 12V from the headlights through the normally closed contacts. When you turn the key on, the relay pulls in, and the circuit to the buzzer becomes the open circuit and will not sound when the headlights are on.
I used allied electronics just as an example since they had all the parts in one place. If you go to:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=330-073
the relay is cheaper, and I am sure you can find a generic buzzer easily to do the same.
No need for a relay.
3M used to offer a kit to do this - sometimes you find them at Walmart or Kmart, I was going to offer them when 3M stopped, but they are so ridiculously cheap and easy that I figured there was no market. You can build your own for less than $4.00.
- Either order a buzzer from Allied (70115780 $1.85) or pick one up at Radio Shack (#273-053 $3.59).
- Stick a couple of spade connectors onto the leads.
- Connect the red lead to the parking lights fuse. The spade allows you slide the lead into the gap between the metal clip & the wall.
- Connect the black lead to a fuse that is only hot when the ignition switch is ON.
- Have a beer & enjoy the buzz.
The buzzer will get power from the lights fuse when the lights are on and the black leads will find ground when the key is OFF so that buzzer will buzz. With no ground (key on) buzzer is dormant.
I like the atari case. That is a great idea.
You should see his computer. 
The buzzer will get power from the lights fuse when the lights are on and the black leads will find ground when the key is OFF so that buzzer will buzz. With no ground (key on) buzzer is dormant.
Knowing my car, letting parts on it trying to find ground on their own will just end up getting lost along the way and wind up at the bar spending too much money on booze.