And it sometimes helps to have a hatchback. If I spend a few minutes loading stuff into my car, it’s enough time for much of the heat to escape before I get in and drive.
OTOH, the problem I had with having a hatch back is that from time to time I’d have (plastic) things melt in there that I never had a problem with in my trunk.
Of course, the ultimate solution is to have a garage. Blazing heat or a foot of snow and your car is still ready to drive.
I put a reflective shade in the front window and try to point the car where the sun will be when I get back into it. It does make a difference especially when grabbing the black steering wheel. Ouch!
I also have vent visors on the front door windows so that I can keep the windows down an inch or so and not have to worry about rain. The vent visors seem to work best if there’s a cross wind blowing through the windows.
Of the two, the reflective shade works best.
VW had a similar feature awhile back,with a funny commercial where a nerdy guy reads about it in his new car’s owner manual and then eagerly shows it to his wife, who couldn’t care less…
…a cat rather than a car. Our cat Archie, of the Sixties, who was colored like the Clinton’s cat Socks or Sylvester of the cartoons, had fluffy fur. He probably disliked the heat of summer as much as we humans–although I don’t know how that works with cats, whose body temperature is three degrees higher than humans’. He seemed to know where all the shady spots were outside; he often stayed in a clump of bushes near our driveway that we called his “jungle.” 
An anecdote that someone might be able to explain. When I was a paper-boy, the weekly collection for the paper delivery was 45c, and in those days, coins were made of silver. Some customers would just leave the 45c on the porch on collection day. Coins in direct sun for a few hours were too hot to touch. Why would the shiny silver surface absorb, rather than reflect, so much of the solar energy?
Shiny or not, if it’s in the sun it will eventually come to absorb enough to become hot. It may even simply reach the same temperature as the concrete (or wood, or whatever) of the porch, as well, though note that you may not feel that surface is too hot to touch. Metal is generally a good conductor of heat, so it may feel hotter to you when you touch it as it more quickly transfers that heat to your hand.
The reason the coins felt too hot to touch may have had more to do with the coins’ thermal conductivity rather than their emissivity. I know Pre-1965 coins weren’t pure silver, but silver does have the highest thermal conductivity of any metal so it can transfer heat to your hand very effectively.
Metals have lowish specific heat capacity - they can’t “hold” much heat energy, so a small energy input/output corresponds to a large temperature change. Thus, for a given amount of sunlight absorbed (i.e. comparing two things the same color) a metal coin will heat up more quickly than (say) soil.
Metals also have a very high thermal conductivity - heat “moves around” within metal easily. Thus, if you compare touching a hot metal to a hot [anything else] at the same temperature, the metal will give you a more severe burn. With [anything else], only the heat in the surface layer is conducted to your skin; with a metal, the heat from the entire object is rapidly transferred to your skin.
They were 9 parts silver and one part copper. On a scale of 1 to 100, with silver being at 100 as the best conductor of heat, copper comes in second-best at 95. (Source: Isaac Asimov.) That would make for mighty warm coins, I’d say.
A/C??? Seems to work well with my black car, white car, red car, tan car, blue car (which I never really had - but anyway), silver car.
I had to drive a black car with a black car for a year in the Sonora desert, and the parking lot at work had no place at all for shade.
The only thing that ever gave me relief was covering the steering wheel with a white towel during the day, otherwise I literally would be unable to grasp the steering wheel enough to drive safely. (We weren’t allowed to crack the windows on company-owned cars, unfortunately, due to high levels of property crimes in the area.)
So much radiated heat would build up that the air conditioner took about 30 minutes to catch up. It was only a 25 minute drive home.
I never did figure out who the hell thought it would be a good idea to order black cars in Sonora.
It surprises me when renting cars in desert areas that agents don’t understand immediately why somebody would want a light colored car. A couple of times I’ve had a car rental agent look at me askance when I’ve said I don’t want a black car. I’m pretty sure they thought I was a racist. But I can’t be, right? After all, some of my friends have black cars.
I wonder what color cars did the agents themselves drive.
There’s a huge opportunity for someone who figures out how to keep cars cooler in the summer. Kids and pets wouldn’t have to die anymore, and your hands and butt wouldn’t have to burn.
I think maybe if the roof and hood were solar collectors there would be enough power to keep the interior below say 90 degrees, cool enough so you could take kids and pets without accidentally baking them.
Pretty substantial. The link below has some specifics, but 50%+ heat rejection is common. You can get clear tints that reject heat, BTW, if interested in keeping the interior cooler but not in the look of tinted glass.
Running it 24/7 must play hell with your mpg.
This. Tint is the best way to reduce interior heat and makes a big difference.
A ceramic film generally provides the best heat rejection, but even a quality non-ceramic film will help a lot.
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So, applying this to the OP, would a metal car transmit exterior-sourced heat to its interior space more efficiently than, say, a plastic or fiberglass car, and therefore be hotter inside??
Other things being equal, yes. I don’t know much about cars - is there usually an insulation layer of some kind inside the panels?