I had to find a hidden register once. I bent the end of some romex at about 6" and shoved it down the duct to find the old opening. I kept checking my progress with a flashlight and mirror. When it reached the hole I could feel it as the wire went past the hole and even felt the wire going over the other side edge. I could see the wire hook the first side as I pulled it back. I marked the wire and then yanked it out to measure it. With much anxiety I drilled an 1 1/2" hole and felt around. I was in the register hole (barely), all was good.
Today is PERFECT!
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