OK what's a good product for cleaning very dirty exterior painted plastic? I have a "birthday" truck (bought a left-over 2018 end of April this year that had been sitting on dealers lot for almost a year), the painted plastic between the grill and bumper doesn't respond to wash detergent and hard scrubbing with a rag. For grins I tried a little CitrusSafe BBQ grill cleaner on a small area, it got about 50% of the baked-in dirt up but still noticeable difference between painted aluminum and painted plastic.

Suggestions?

 

Klasse...all-in-one.

 

Wow!
The plastic looks almost like it is discolored (or a different paint color).
I have seen many instances where the painted plastic is a slightly different OEM paint color than the metal.

I would be careful using harsh chemicals to clean the discoloration as that may damage the paint.

Although, it is apparent from the pictures that the plastic is a different color it's hard to determine what/why from a pic.

As with paint decontamination there are two ways to do decon. (Chemical and Mechanical)
Chemical (via IronX) and mechanical (via claybar) the same may be true of this area.

I might suggest trying a few different APC products full strength (including Simple Green) - nothing harsh.
If that does do it then polishing the paint (via machine) may be the way to remove any surface funk bonded on the paint.

IMHO unless you are familiar with DA machines, pads, polishes etc. then I would have a pro detailer physically look at it.
They may have a solution. It won't cost you anything to have it looked at and get an opinion.

Let us know your results...