short of replacing these altogether, can anything be done to improve their appearance??? mine are quite scratched and dingy. 99 528i

You can polish them. Wolfgang has a plastic lens cleaning kit. http://autogeek.net/plastik-lens-cleaning.html

They sell headlight cleaning kits that consist of several super fine grits of sandpaper and some polishing cream. I used it on mine and it made a big difference. You have to do it a few times and it will clear them up.