Condensation problem destroying headlights

I have a problem with a 1997 Oldsmobile Delta 88 that is now in excellent condition except for the headlights. I have noticed that there has been condensation inside the plastic of my headlights and apparently also the red tail light on the drivers side since the car was repaired three years ago from an accident. The plastic on the headlights is no longer clear and seems to be becoming opaque. A few weeks ago I when looking at my tail light on the drivers side I also noticed condensation there. That whole unit (tail light)was replaced in the repair job done by the Oldsmobile dealership body shop along with the repairs to the front and side of the car three years ago (significant damage on front passenger side at and in front of wheel and some at left rear driver's side). There is no condensation on the rear tail light on the passenger side that was not damaged in the accident. The dealership told me that the headlight assembly may need to be eventually replaced in order to pass inspection. I have two questions. Did the body shop do anything wrong in the repairs to cause the condensation to form inside the headlights and tail light? How difficult would it be to replace the headlight assembly and could this happen again even if it is replaced?