The number one culprit is, you guessed it, the SUN. Think of it as a type of cancer; way back then those old cars just didn't know the dangers of constant exposure. In addition to temperature extremes, UV light plays a factor. Over time, UV light can break the chemical bonds of a polymer, causing degradation. The major problem, however, is that the dashboard just plain dries out. Polymer additives called plasticizers dissipate over time.
These are materials added to a polymer to lower the
glass transition temperature. Compare a
length of rigid poly(vinyl chloride) (PVC) pipe
to a shower curtain, which is also PVC. The plastic curtain has added
plasticizer to make it soft and flexible. Plasticizers are usually small
organic molecules. Bis-(2-ethylhexyl)phthlate, DOP for short, is commonly
used with PVC. DOP, by the way, is one of the chemicals that contributes to
that "new car smell".
Now guess what? The outer covering of your dashboard is made of PVC. Over the course of many years in the summer sun, the plasticizer dissipates. The result is...  well, you know.
Dashboards are Complex
The dashboard of a car has three layers.  You don't see the bottom layer which is a glass-fiber reinforced thermoplastic. On top of that is a layer of polyurethane foam. The top layer on old cars is, of course, PVC. More recently, dashboard coverings have been made of PVC blended with something called ABS. ABS is an acronym for a block copolymer made of acrylonitrile, butadiene, and styrene. PVC blended with ABS effectively does the same thing as PVC with added plasticizer, but in this case, the ABS doesn't evaporate so the dashboard lasts a lot longer. Plasticizer is still added, however, because consumers seem to like that new car smell!
PVC-ABS Care and Maintenance
There was once a debate over the effectiveness of the product Armor-All . One assertion was that, instead of replacing softening oils, Armor-All actually brought oils to the surface causing them to evaporate more quickly. Of course, another study showed the complete opposite. Vaseline is favored among many automobile enthusiasts. It's cheap and shows no negative attributes. Somewhere along the way someone came up with the idea of using sunscreen. It seemed reasonable enough at the time, after all it blocks harmful UV light, right? Problem is, sunscreen doesn't preserve or replace oils, and it leaves an undesirable residue.
The ultimate solution: park your car in a garage and only drive at night! (just kidding!) The best solution is to become a polymer scientist and develop better materials.
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