My theory - based upon absolutely nothing but wild speculation — is that the fire started due to careless smoking by one or more of the passengers.
Freebasing?
Flame retardant materials have been required in vehicle interiors for decades. That doesn’t mean it won’t burn but it’s very difficult to ignite under any reasonable circumstance such as a dropped cigarette. Something served as an accelerant. Flammable liquor?
Total speculation here also, but a tipped over bottle of alcohol and a cigarette lighter would be a major problem too.