Time alone changes the plastic in some ways. Some people use treatments available in many places. I forget the names but others will chime in .Some of those treatments have less than a stellar reputation. There are a couple though that people think highly of.

Best the car not be left outside with all the windows rolled up on a sunny summer day. Even with a windshield protector although that helps a lot.

I expect as the plastic ages it becomes ever more touchy about the thermal expansion and contraction it continiously undergoes. I think the long term condition is reffered to as becoming brittle. Plastic perhaps initially outgasses heavily during the first few years. Then the outgassing declines a lot but always will remain a little.

Since even the interior of a sealed car can easily exceed 120 degress as far north as I am is not good. I would not like to see what is possible in more southern climates. Plus the length of all these factors is shortened as the number of hot seasons exposures are compressed by the additional length of the hotter summers south of me.

That is why if you want a good dash used replacement the further north it is sourced the better the chance of getting a good one and it lasting longer.

One solution I thought of awhile back was to make some kind of a ventilated suroof insert. It could be kept in the trunk and used to secure the car when you left it out on a hot sunny day. It should be easy to construct and make secure with a little imagination. This would keep that overall interior temperature down somewhat. That would possibly be also benificial to the seats as well.

Primarily for our dash pads the uv component of the sun may be the worse issue by far.