1990 GMC 1500 Pickup Q&A

1990 GMC 1500 Pickup Question: Brake lights go off when I turn headlights on in my 90 GMC Sierra.

My brake lights go off when I turn the headlights on in my 90 GMC Sierra and the rear parking lights do not come on when I turn the lights on. I've replaced the headlight/parking switch and replaced the bulbs. They still don't come on. Any idea on what to do next? -
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