You should clean the dash using a diluted apc (1 part apc and 30 parts water). 3M apc (all purpose cleaner) is available at shahid autos, F-10 (near shell pump). This should clean if there are any spots due to something sticking on it. If its the plastic that has given up (due to other factors like heat or placing something on it causing a scar), then ofcourse those can't be restored.
Once cleaned, you should protect it. For protection, get yourself a Meguiars ultimate interior protectant . A better way to appy it would be to spray it on the towel and then spread that on the plastic. This will avoid splatter on the glass.
Hope this helps...