Hey, you guys might want to stay away from the brake fluid... I tried that with a resin '66 batmobile body and it took a year for the resin to firm back up again afterwards. It turned the body flexible and rubbery and actually ate through in some spots - the body was completely ruined.
I use Easy Off oven cleaner. Place the parts in a ziplock back, spray liberally with Easy Off and then close up the bag and let it sit for a while (I wait over night). Scrub with an old toothbrush to get in the panel lines and corners and such. Sometimes a second application is called for, but I've never had a problem with damage to the resin.
Some folks swear by Castrol Super Clean too...
Mark Snyder
Seoul, Korea