Q: Rear lights & Brake lights won't work on 1990 Chevrolet 1500 Pickup

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It started with no right rear turn signal and no brake light. Light panel is good.I know this because I put it on another truck just like mine.New plug was put on.Than the left side rear also went out. All fuses are ok, along with bulbs.Power from front coming back on left side(driver) of truck has a connector where it splits off to both sides.There is a brown wire,two green wires(two different green colors)and a black wire,for the right(passenger) side.Tow wire pkg was already disconnected but wires are still connected to power connecters.The four prong power connector coming from electric up front has a brown wire,white wire, and two greens. Coming out of four prong the white goes and splits to tow package and rear drivers side lights.Rear driver side lights has a black, white,brown and green wire to it.Passenger side has brown, black, and two green wires.White wire on four prong power is loose and when moves around blinker will work, but left side won't work at all.Front blinkers fine.We even replaced the two big square flasheR fuses.How do I get these lights to work??? NO REAR BRAKE LIGHTS EITHER...
(1) Answer
(2) Comments
You will need to check the wiring going to the back using a test light and a wiring diagram. Test the wires at each connector starting in the front. You will need to find the spot where power or ground is being lost a repair the poor connection or broken wire. Some basic wire colors are as follows: Green - turn signal one side, Yellow - turn signal - other side, Brown - park lights, Black ground.
same problem here with my 1989/90 1500.
left side: no brake light, turn signal
right side: no back up light
weird shit!!

Replace the rear tail light Circuit board. It's behind the right rear tire above the exaust pipe, 4 connectors going to it. Mine was gray, has the color of harness on it My right rear light didn't work,2002 Avalanche, replaced it for $35- $38