'68 B'cuda inner (plastic) grill detailing tips?

I'm back...love the coffee mug with your Cuda Pic! (LOLed) Going to attempt to attach pictures of my grills.
first 3 are of the Non-chrome with tar/road grime?
-after "chrome" shot, shows "spring special" grill... with white paint over-spray, and film of road grime(?) or what-ever. that needs to be cleaned off them to bring them back to a decent shine. these things are 50 years old so not expecting miracles, but think the will clean up nice.

thanks everyone! hope there are some tricks up some ones sleeve to detail these without wrecking them!