On my 86 300sdl, only the driver's side started to get darker. I took the entire lens apart and cleaned the lens, put it back together and it lasted about one year. I will have to do it again. I personally dislike the clear lens look on cars. I think Lexus made a big mistake when they put the clear lenses on their RX 330's.

On another note, it always bothered me how the rear of our cars seem under lit at night. I scrounged some parts from some old rear lights and put a socket in the fog light lens area. I placed the specific wattage bulb in their and now the rear lights were too bright. I then replaced those light bulbs with a wattage one half of the recommended fog light, and they look perfect!!
I have two extra lights now as running lights in the rear. It really brightens up the rear end at night.
John
1986 300sdl 97,000 miles
1985 380se 156,000 miles
1967 VW Beetle 148,000 miles
2003 Honda Odyssey
2000 Toyota Camry