Assuming previous owner saw this but didn't fix it, or else I am very lucky. Light comes on when lid about halfway up, then goes off before full open. Comes on about halfway down, then goes off when closed (verified inside trunk). New light didn't help. Only part on GM diagram is light connecting to one wire. Isn't there a mercury switch somewhere? Have had the lid liner off twice working with balky back-up light, but didn't see anything connected to trunk light under it. What am I missing??

 

Yes, not sure if allowed to use Hg anymore for anything but used to be allowed for all types of switching. Lasts forever, sounds like if switch isn't integral with lamp it is not secure. Will drain battery if somehow it goes on when trunk is closed. Probably switch not much larger than wire itself and could be on grounded side or maybe intermittent ground itself to trunk lid?