Cracks in dashboards have several causes.
Interior components like dashboards are usually made from polypropylene (i.e. PVC) which has characteristics like thermoplastic (i.e. ABS).
Polypropylene will degrade or oxidize when exposed to heat and ultraviolet light causing shallow crazes to form on surface. Magnified crazes have pattern like crow’s-feet or chicken scratches in dirt. Eventually, these will worsen and form cracks in material.
The rate of degradation is a function of area, exposure to sun (temperature and UV), and oxygen present in the environment – the passenger compartment. Consider 1995 Chevy Camaro with sloped windshield and “huge” dash-top cover area.
Exposed to heat and UV from sun (i.e. Florida), poly oxidizes and alcohol (hydrocarbon) and acid are released both of which have a distinct aroma.
Even with protection, Camaro dash-top cover becomes brittle. After ten years of exposure cracks form and eventually the cover crumbles into small pieces.
Another culprit, that accelerates the process, is ozone - reactive form of oxygen. For example, with windows rolled up on hot day, hydrocarbons released from dash and top cover mix with oxygen in passenger compartment to form ozone in the presence of sunlight.
Ozone degrades rubber, nylon and fabric dyes. It causes colors to fade and elastic materials to become brittle. Other evidence of ozone formation is the haze (smog) that forms on the inside of front windshield several days after cleaning it.