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2010 Mazda CX-9 failed inspection for a dead high-mount LED 3rd brake light. I bought a new light from the dealer - it doesn't work either. Checked voltage coming from the connector into the light - steady 4.5 V when brake is depressed. Wired a 9V directly to the old LED - lit up right away.

So the problem seems to be low voltage coming into the light. Not sure how much I need, but 4.5V isn't enough. What could cause this? What should I check? Where should I even start looking?

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