Could also be that the lamp housing/mercury Holder is gone bad?
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Easy to test the switch. Simply remove it from the spring clip holder and rotate it while checking the continuity between both leads. If it makes and breaks (as reliable as gravity if the mercury is still inside the tube), it isn't the switch.
You are lucky! 9 out of 10 mercury switches of any era, even early 90s (such as k1500 trucks, etc.) don't work. And of course, mercury is frowned upon these days so the replacements are mostly purely electrical. But definitely keep an eye on it from time to time. If you start running into battery drain issues keep checking that trunk!