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If your button is malfunctioning you can probably open it up and clean it with contact cleaner.
Are you sure it's not just damaged wiring though? I would definitely unplug the harness and verify that the button circuit opens/closes with a digital multimeter.
Pins 6 and 4 should be open normally, when you press the button those pins should be closed. If you're not sure how to test this with a DMM just holler and I'll explain it.
Also, verify your lockout button in the trunk isn't turned on accidentally.
If you are physically MISSING the button I would try to find either an eBay used tail light or possibly an aftermarket one? that uses the same button and retrofit it to work. It's all integrated into the tail light I believe and not a separately replaceable component.