I am not sure about RC car plastics, but for standard ABS plastics, immersing in Salon 40 with either UV or natural sunlight works wonders. The yellowing (or browning in extreme cases) is the bromine fire retardant separating from the plastic and rising to the surface. Salon 40 provides the oxygen and UV light the "excitement" or movement of the molecules to rearrange themselves.
I posted this because many of you have old 1:1 cars as well as old RC cars

