With odd reaction such as you are seeing I would look at all of the grounds. From there I would start at the tail light assembly a inspect for corrosion in the wiring connections. Depending on how it is wired, the brake lights and turn signal could be the same conductor.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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