I have this "rain tray" from my 81 Porsche 928. This is the thin, hard plastic, but flexible part that covers up the windscreen wiper mechanism, ahead of the windscreen but behind the engine bay firewall. Mine has these white spots on it that I have never been able to clean off. Its either spray for coolant or perhaps spray wax . how to try to clean this
you can try an all purpose cleaner and agitate with a good detail brush. then dress it with a nice vinyl rubber dressing. Adam's APC Adam's In and Out Spray Auto Finesse Hog's Hair Brushes should do the trick. you can also use Pre Wow or Black Wow