Fox2, USA, has published a story warning the public that a water bottle left in a car in the sun can be a fire risk. The article quotes however the Getty Conservation Institute: “seats are designed to be fire-retardant and self-extinguishing, thus your car seat is more likely just have a burned hole in it than to actually catch fire.” Most car manufacturers worldwide respect the US standard FMVSS 302 (horizontal burn test) for car interiors, including seats, arm rests, panels, floor coverings, etc. Although this is much less demanding than for example UL 94-V0, it may indeed (as suggested) be sufficient to prevent a fire starting from a point heat source resulting from sun through a bottle.
“Warning: Leaving bottled water in your car could start a fire”, Fox2 Detroit, 18th July 2019 http://www.fox2detroit.com/news/local-news/warning-leaving-bottled-water-in-your-car-could-start-a-fire