Check your grounds in the rear. The current flowing through your brake light filament will follow the path of least resistance to ground. When your ground isn't good it will follow the path through your tail-lights, into the harness, and into the parking lights. It's not an uncommon issue.
Cj probably has it solved for you. One other thing that I saw once was a taillamp bulb that had a broken filament. The stop filament was broken and leaning against the tail filament, causing a path for brake light current to get into the tail circuit. I admit that is a bit of a long shot.
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