Hello. It was about 85 degrees Farenheit outside. Some medicines I’d purchased stayed for about 4 hours in the rear compartment of my silver 1999 Honda Civic. I’ve read that temperatures in the passenger compartment can reach 140 degrees on a sunny day, though the rear compartment does not heat up that much. Does anyone have any data or estimates of how hot the rear (luggage) compartment might have gotten? I’ve googled, but most webpages mention the temperature in the passenger compartment. Thanks in advance.
By “rear compartment”. I meant the trunk. Thanks.
I don’t know how hot, but the trunk sure can get very hot on a parked car.
Call your pharmacist (chemist, if your are Brit). Some medicines are more sensitive to high temperatures that others. You could put a thermometer in the car’s trunk (boot) and leave it for an hour. After an hour, open the trunk, and immediately look at the thermometer. You could give that temperature to your pharmacist.
Yes, it’s possible that an enclosed car on a sunny day could hit 140 or close to it. Much depends on the locale, amount of glass and angle, etc.
I routinely carry an AC thermometer in my car and have noted many times that it was on 120-130 when I first got in.
Many years ago (1980) here in OK and TX we had a heat wave of 100+ degree days that lasted a month.
The local news story one night showed a lady who cooked a large roast for dinner that night by wrapping it in foil and placing it in a pan underneath the rear window during the afternoon.
She also fixed baked potatoes to go with it. 
That should give you an idea!