trailer lights on 2000 f-150
If they were working previously check under the vehicle at the connections point, especially if the plug is aftermarket. First, use a multi meter and check for voltage on the female plug. Water and moisture can get in over time and cause corrosion.
Checked all the fuses but the ultimate answer was to replace the old (and I might add corroded) plug with a new one. Soldered the connections and the LED lights on the new one light up. Would suggest anyone considering a new on get one with LEDs - makes it easy to check out power when hooking up the trailer.
I know this is an old question but, I had this same problem on my 2000 F150 Work Series Extended Cab where no brake light at 7 way trailer plug on truck. I just spliced in to drivers side brake light wire and wired it directly to 7 way plug for brake lights on trailer.
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