The taillights on my U320 were intact but badly faded with many scratches, one had been gouged by a grinder or sander at some stage of its life. Most of the scratches and a grinder mark were sanded out with 400 grit wet and dry paper then finshed off with some 1000 get wet and dry paper then buffed up using a cloth buf, rouge and metal polish.
The faded amber sections were then treated to 3 coats of transparent amber paint that is designed for faded tail lights. After 48hrs this was buffed up to a reasonable shine. When applied it often looks a bit too dark and flat but it thins out a lot as it drys.
The end result is not perfect but it looks a lot better without the faded and scratched lens plus the bulb holders and wiring is a lot more reliable.