I pulled the rubber boot over the wiring loom free and found damaged wires like the mechanic described but it didn't affect my trunk lid. That was from water in the switch. I removed the rubber boot and it worked fine without the boot that ironically held water in. With a proper scanner you can read the button press values real-time to help determine where the fault lies.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)