how do i remove my 67 galaxie 500 coupe rear seat to add 3rd (middle) seat belt

check your pm box.

Move both front seats all the way forward to make room. If you step into the foot well and face the rear seat. bend down and locate the bottom of the rear seat near the carpet. there is a metal bar that is along the bottom of the seat. you will need to push it back toward the rear of the car and then while holding back lift up. Do this on both sides. Some times pushing back and shaking it up and down will make it pop out. That lower bar locks into a slot in about the middle of the front seats so if you push in that area the lower seat will come right out after you get both sides out of the slots.