We were looking at the rear light system to put a quick connect trailer lighting

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Use it to TEST each and every fuse you can find on your vehicle! There is one that's shot somewhere!! it may not even be labeled as you think, best to test all of them.. Have fixed a bunch of electrical problems this way and that the fuse had already been checked!! Light should illuminate when each side of each fuse is touched. Hope the fuse is 'your' trouble..I really think so!