Cleaning old Plastic

[Jim Rowan <jimrowan@uic.edu>]

The plastic exterior of my Smith's heater blower assembly 
has a dirty brown marbling hue to it that I can't get off.
The voltage regulator cover looks the same. I've tried plastic 
cleaners/mild abrasives but it didn't help. Maybe it's just 
the plastic ageing and nothing can be done about it short of
painting it black.
Anybody have any tips for cleaning it?

TIA

Cheers,
Jim Rowan
60 Sprite