I have a similar problem with my under hood lamp. The mercury switch is like the one pictured in your post. However when the hood is up the light is off, down and the light is on. I thought maybe the mercury switch was inserted upside down. So I changed it and put the tit on the switch down and the rounded end up. Now the light works all the time regardless of the position.
I didn't plan on making it operational but after seeing how neat it looks I want to. I do not want a toggle switch however.
I am not concerned about draining the battery as I have a master cut-off switch and anytime the truck is parked/stored the switch is off.
