Ok, on the brake pedal pushrod that goes from the booster to the brake pedal is the brake light switch. In addition to controlling the brake lights, its signal is used for the shift interlock (see attached)
Running along the length of the top of the switch is a retaining clip that can break easily and cause the switch to slip of the rod somewhat and no allow it to work properly. You need to check that and replace if broken. If, then you need to make sure you have the harness connector securely attached to the switch and no broken wires. Secure as needed and repair any broken wires.
If above ok, then the switch has failed and will need to be replaced. It runs around $15 at any parts store
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