This is a taillight adapter which will convert your two wire system on your towing vehicle up to a three wire system to your towed vehicle. Certain applications will be say, your RV has a four wire system on it or maybe a five wire system converted down to a four wire but your towed vehicle has separate turns signals and brake lights. You would use this box to convert it back up to that at the rear of the vehicle. See a few technical points on how it actually gets installed in this video.
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