Rear blinker not working properly

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These light share power and ground at splices. I suggest checking the wiring before the socket with a volt meter to see if there is 12 volts with the signal on. If there is then check continuity to ground by using the positive probe for the orange and light blue wire and negative probe to the black wire at each socket. If it works when you do this and use a goo chassis ground for negative probe but not with the black wire it's a ground issue.

If voltage is not present on the orange and light blue wire check the splice that is near the bulb wiring. It's not uncommon for solder joints to break here.

I'll post the wiring diagram below. If I can help further just reply back.