Let's deep dive into the physics of the problem.
A location in the car is not enough, because the entire car will tend to equalize to the same temperature over time, and it will be quite high. Car interiors can be 60-80°C (140-175°F) or more because of solar gain, the interior being basically a greenhouse. It's no joke. I had a set of styrene paint tray liners crinkle up simply from sitting in the car. I also had a phone overheat from direct sun and found there was nowhere in the car I could put it where it would cool back down to the 105°F/40°C it needed to start working again. (my car didn't have A/C).
Food spoilage is also serious business.
So you really need a 3-layer defense. And we'll get into the thermodynamics of it shortly. By the way, I'm sorry to introduce the clunky "BTU" unit, but it's ideally suited for what we're about to do.
A BTU is the heat needed to raise 1 pound of water 1 degree F. (or 0.25 kilo/litre of water 1 degree C, that worked out nice). Or 1050 joules.
Water has the highest thermal density of any material. Thermal density is weird - because heat is not related to mass/weight, it's related to electron state in atoms, and corresponds to number of atoms more than anything else. Uranium atoms are 238 times heavier than hydrogen atoms. So the thermal density (heat needed to raise 1 kilo of material 1 degree C or F) varies quite a lot among materials.
(sort of explains how "The One Ring" could come out of a fire and be cool; it probably was made of a very high atomic mass material).
Let's say your sandwich weighs 0.25 kilo (7-8 ounces) and has half the thermal density of water. So 1 BTU will raise your sandwich 2 degrees C. 15 BTUs will raise it 30°C, and now bacteria are thriving and spoiling your sandwich. Not good!
Suppose you have a 10 watt cell phone charger block, you notice it gets a bit warm, call that 2 watts of internal inefficiency. That is 6.8 BTU/hour. So not a lot of heat.
1. Keep it out of the sun.
The source of all this "solar gain" in a car is the sun. The sun's rays output 1000 watts (3400 BTU per hour) per square meter. You may notice how quickly phones overheat when in the direct sun. So a shady parking spot helps a lot.
And placing the cooler in a shaded spot inside the car is essential; if the cooler is in sunlight it will be overwhelmed (and covering the cooler with a shader is fairly pointless if the heat from the shader will simply contact the cooler).
Believe it or not, the color of your car matters. Each paint color has an albedo or percentage of solar energy reflection/rejection. White goes as high as 0.91, so 910 watts reflected, 90 watts absorbed. Black has an albedo of <0.1, so 100 watts reflected 900 watts absorbed. (absorbing light is practically the definition of black, after all). Yikes!
2. A cooler.
Just a basic "highly insulated" styrofoam or other type cooler. I use very inexpensive ones molded out of raw styrofoam. This will not stop the entry of heat. But it will slow it down enormously, to the point where where the thermodynamics starts to become possible. If it's away from direct sun.
The problem is, with such a small mass (15 BTU ruins our sandwich), the cooler is good, but it's not the endgame solution.
3. Add thermal density.
Let us suppose hypothetically that we cannot prevent 100 BTU of energy from entering the cooler. If the sandwich is alone, it is dead.
However. Suppose there is also 10 kilograms (liters) of water (or 20 pounds) also in the cooler. How does this change the physics?
Now we look at the water and the sandwich as a single thermal material (it's dominated by the water; the sandwich is rounding error at this point). Remember 1 BTU warms 1 kilogram (liter) of water ¼°C (or 1 pound 1°F).
100 BTU warms 10 kilos of water 2½°C. (or 20 pounds 5°F). Well, that's not bad at all.
There is the key to it. Stuff the cooler with as much water as you can in bottles, to moderate thermal changes.
4. Let's use a physics cheat code!
Changing state (solid from liquid or liquid to vapor) requires a certain amount of thermal energy just to make the change.
If you have 1 pound (0.45 kilogram or liter) of ice at 0°C (32°F) and you add 144 BTU, you have 1 pound of liquid water but it's still at 0°C (32°F)! You had to pay 144 BTU simply for the state change.
So to absorb 100 BTU, we don't actually need 10 kilo or liter of water. We only need 0.3 kilo or liter of ice.
That's much more practical!!
However, if you don't have the thermal density of water I mentioned in part 3, once the ice melts, the cooler contents warm quickly.
My perfect ice container is soda bottles (they're tough) filled 90% with water and then frozen horizontally. After the ice depletes, the water's thermal density still slows warm-up. And the bottle stops leaks.
A personal aside... I cross the USA a lot by car. A "large" fast-food soft drink is 32 ounces (1 liter) and comes in a common cup. When I travel, I keep two of them in my cup holders. When I stop for fuel, most travel centers/truck stops (Pilot, TA etc.) let you refill your cup with ice for free*, and I do. A 32 oz cup gives me about 16 oz of melted ice, so 0.5 litre. Once such cup lasts hours in my cooler.
* It works smoothest at truck stops, but any convenience store with a fountain will sell you a cup of ice at sane cost. You can also ask at fast-food restaurants (really helps to buy something first).