your headlive cover's plastic is degrading due to the UV (mainly) rays from the sun causing the plastic to deteriorate, and become cloudy and more brittle.
You may want to invest in the cheepest ZKW headlights. If you dont get the DDE, HID, or any other fancy stuff, they arn't too expensive.
If you really want to cheep out, and I sugest you dont, you can get the DEPO elipsoids, the protective glass is thinner so it has more potential to break if a pebbel hits it. They also have a tendancy to fog during large humidity shifts, such as heavy rain or after a carwash. Ask el presidente, he has them. He's happy with them (as far as I can tell), but I wouldn't be.
Check out Umnitza
, they sell ZKW, DEPO, and apparently now BOSCH, we havn't seen or heard of those yet in these forums, so most of us can't really say anything about those I don't believe.
BTW: you can't open stock american headlights, they are fused shut.