Couldn't really figure out how they worked because I couldn't find a switch anywhere. the '96 service manual doesn't mention anything about how it works, just where it's located and the circuit info. I never really bothered with it until my light stopped working. Bought a new bulb, still nothing. Don't know how to use a multi-meter, so had a hard time with diagnosis. i figured I'd just go to the junkyard and get a replacement harness. Apparently, these things are like gold, because every yard I've been to, and literally hundreds of full-size Ford trucks I've seen, all have them cut off.
A friend thought that maybe there is a mercury switch inside and then I wondered if the EPA requires the yards to remove them (I'm in the environmental field, so whenever mercury is mentioned, hair on the back of my neck stands up!)?
Lastly, since I was determined to figure this out, I wondered why haven't I seen any mods on these forums for bigger and better lights under the hood? Seems like for those that are good with circuits, electricity, etc, this would be an easy mod.

 

It does use a 'mercury' switch, by which I mean it senses when the hood is open by the angle, and turns the light on. Whether or not it has actual Mercury inside, I do not know.

 

The light in my F150 will not turn on unless I open/close one of the doors first. Then the under hood light will stay on for 5 min or so.