There are two bolts inside of bed unbolt to remove taillight. Remove bulb, push down slightly and turn. Replace with new and reassemble.
on the brake pedal
check and see when your brake pedal is not pushed down if it is pushing the brake swith pin back in if so you need to replace the brake light swithch located behind the pedal
3rd brake light or rear brake lights?
disconnect it from your brake pedal lever.
The brake lights in your 1983 Nissan pickup truck are controlled by the brake light relay switch. When the brake light relay switch malfunctions the brake lights will stop working.
Those tail lights could be the brake lights you are seeing and that would be a faulty brake light switch or the brake light switch is out of adjustment.
Cut it off and replace it with a new one.
how to replace lower ball joints on 1995 4 wheel drive Nissan truck
Drop the tailgate, pull back the bed cover on the corner behind the break light you want to replace. There are bolts back there that hold the light in place. Undo those bolts and the light will come out easy. Replace and tighten bolts back and you are done. Very easy to do with some mechanical skills.
I have an 87 Aero Coupe and my 3rd brake light bulb is in my truck. I think yours would be the same too.
brake calibers might need replacing,the pistons might be getting stuck,its easy to replace.
you will have to replace.