Steven, the marks I presume are cloudy/white wax and polish marks, yes?
If so, you need to remove these before following the man aboves advice and protecting/dressing the plastics.
The best option I have found (through testing numerous myths etc) is WD40 and a lot of elbow grease with a MF cloth. The WD40 breaks the polish down and the rubbing removes it. Only once the plastics are clean can you then protect them and insure they look new for longer.
Hope this helps.