It's probably a common ground for the ECM. Alot of these switches are originally ECM controlled. They do not carry high amperage. Just milliamps.
Also the coil side of a relay uses only milliamps to activate. Takes almost nothing to turn them on.
Maybe the way you're testing is slightly wrong. I would try wiring up a relay and see if it will actuate the relay. And if it works move one to wiring it up permanently.
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