transmission with Battery removed I can shift, with battery in I can't

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My car was diagnosed with a bad brake switch the car drive anyway and then had an accident not related at all.. but the damage was to the under carriage oil pan area and since then my shifter will not move from park with battery connected with battery remove the shifter works fine So I think the brake switch is bad and the foot on the pedal light is on by the shift lever .. So how do you replace this switch I hear it's tricky with the plunger ? Can anyone give assistance!!! It is appreciated Have a Happy Thanksgiving..