Lights are generally 12 volt bulbs. When you plug into AC, the inverter provides power to charge the 12 volt system. If you have no lights when not plugged in, the batteries need charged or are bad and won't take a charge. Its probably a good idea to change the batteries if you don't know old they are.
The plug usually is added to the drivers side of the bed, so the cords are usually about 8 ft or so. It is to control outside markers, turn signals, etc. Usually separate than the 12 volt system unless setup to charge batteries while driving.
'10 Ford F-450, 6.4, 4.30, 4x4, 14,500 GVWR, '06 Host Rainer 950 DS, Torklift Talon tiedowns, Glow Steps, and Fastguns. Bilstein 4600s, Firestone Bags, Toyo M655 Gs, Curt front hitch, Energy Suspension bump stops.
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