Nearly all polymers outgas - what we know as "new car smell". Over time outgassing causes the polymers to shrink, which builts up stresses and eventually causes cracking.

The key is to seal up the surface and one of the best products to accomplish this is Pledge Furniture Polish, and the original unscented type is best if you don't want your car to smell like a lemon.

Pledge is basically carnuba wax in a petroleum distillate emulsion. Just spray it on and wipe it around until the volatiles dry. It will leave a nice satin finish.

If you have to regularly park your car in the sun buy a cover that is made from a material that is a good solar reflector. Tyvek is great.

Duke