[JP_79Formula]

79 Formula: I found a loose light bulb in the trunk, it looks to be
factory wired and has 12V with no doors open or lights switched on.

Is this factory?

I suppose it would be mounted so that when the trunk is closed
the mounting bracket is isolated from GND and it's an open circuit
so the light is off. When the trunk is open contact is made to GND
and it illuminates... If so, how is it supposed to mount? Thanks.

[PMDJIM]

It is indeed the trunk light. You need to disconnect the lamp portion at the plug in photo. Then the wire must be "fished" through the deck lid to an opening at the rear of the deck lid. If I had a camera i'd take photos.

[Grand73Am]

I took this pic of a t-lid I have handy. The lamp wire is fed through the hole on the driver side of the lid, following the route of the green line I drew at "1". The lamp bracket screws on at "2".

The lamp works off a gravity switch inside the lamp. I don't know if these lamps are a mercury switch or some kind of moving ball or cylinder inside it that makes the contact. When the trunklid is the up position, the mechanism in the switch moves to a position that makes a ground connection, and light goes on. When down the mechanism opens the ground and the light goes out.

[JP_79Formula]

Thanks Jim & Steve!

I had no clue how to run the wire and where the bracket mounted.

A position/gravity actuated switch sounds dicey... like it would
tend to stay on all the time if some dirt or corrosion got in the picture.

Guess I'll try to hook it up and see what happens.

[Grand73Am]

You're welcome. The switches are pretty dependable. They've been in wide use for probably 60 years or more. I did have one that stayed lit when I closed the trunk in a 67 Impala that was a little rusty. I never investigated...just took the bulb out. It might have been okay if I'd taken the lamp off and shook it back and forth and reinstalled it.

[GT182]

I had the trunk light in my 79 Formula. I'm pretty sure it was a mercury switch that activated it. Same for the light under the hood.

Steve's photo shows correctly how the wiring runs and the light's location.