I don't know how true it is, but I've read that it's not a good idea, the theory being that the grease that is used to lubricate internal parts of camera and lens, can basically melt if the car interior gets too hot, that can cause the lubricant to not only run off of parts where it is needed, but it can run onto parts where it will cause problems..
that being said I agree a car trunk doesn't get nearly as hot as the car interior, I have a black car with fold down rear seats, and the trunk still doesn't get as hot,
what I've done already in a crunch situation is to use large garbage bags, I put a bag or 2 of ice in a garbage bag, twist it shut, then put that and my camera bag inside 2 large garbage bags and close that, there is always the risk of water and the camera bag meeting , but so far I've never had an issue and the camera bag stayed fairly cool..
if I know in advance I use the blue gell ice pack things...