Had that exact same issue one time. I felt like an idiot when I figured it out after hours of “troubleshooting”.
Its funny you mention the simple things. I purchased an 87 Regal about three months ago. Right turn signal worked normally. Left signal would make all lights flash as if hazards were on. When I pressed the brake pedal, the front parking lights lit up. I was thoroughly confused.
End result was a single filament 1156 bulb in the left rear housing where a dual filament 2057 was supposed to go. Changed one bulb and all my problems went away. Now I can safely turn left without confusing everyone! Lol
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