When putting car in reverse, the view on the dash shows the yellow lines, but the rest of the picture is foggy. Is the lens dirty or what? Ans where is the lens and how to clean it.
That is almost certainly what it is. If you stoop down behind the vehicle and look at the places you might find an overhang (Plate area, lift area) you will see it there. A bit of water on a paper towel is what I use. Be gentle.
The lens is likely located somewhere above the rear license plate.
A few times each year, I clean mine with Hope’s Perfect Glass.