The turnsignal and brake lamp circuits are completely independent, so it's not a turnsignal switch problem. I'm thinking that what may be happening is that there is a problem with the tail lamp grounding, which is causing the stop lamp to ground through the turn signal bulb.
The tail lamp assembly is grounded through special mounting nuts that have sharp edges designed to cut through the paint. If you car has been repainted recently these nuts may not have done their job.
If the rear lamp assembly does lose it's ground, the stop lamp current can find a path to ground by backfeeding through the turn signal lamp, or side marker lamp filaments, then through the harness to the front lamp ground, which is hard wired.
It's always worth checking the rear lamps any time there is a strangeness occuring in the TS or stop lamp circuits.
good luck
Kelvin Dodd
Global Sourcing Engineer
Moss Motors, Ltd.
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