We have a 1997 Camry and a 1994 Mercedes C280-- we first noticed a weird chemical smell (like acrylic nails or melting plastic) coming from the front right (near the battery) of the Toyota. It would only happen when the car was really hot- we live in Las Vegas, so driving for a long time in 107+ degree weather. AC continues to work just great. Found out the battery was bad/leaking- so we replaced it. New battery is like 2 weeks old, and when the car gets really hot, we are still getting the smell. We have a very slight oil leak, but nothing else. Now- to make it even more bizarre, the C280 is giving off the same smell in the same situation (driving 20+ minutes in high heat), although from a different area of the car (more to the rear of the car). We can sometimes smell it in the cabin if the air is on, and the smell will stink up the garage bad enough that we put the car (whichever was driven) out on the street until the car cools and the smell dissipates. Any ideas???
When the battery leaked…did you clean ?
Use baking soda and water to clean well where anything may have splashed or run down.
Where is the battery on the Mercedes ?
Battery leakage was very minimal- was mostly bulging of the battery. Haven’t cleaned it as there really wasn’t much to clean (according to my husband) 
The battery on the C280 is in the rear, but on the opposite side of where the smell is coming from. (Smell from rear driver side, battery rear passenger.) We have also tested both the new battery on the Toyota and the alternator to ensure it wasn’t an overcharging situation. It is weird that it is only when it is really hot out/ the car is really hot- (Cars were in FL prior to us moving to LV, so still was hot, not quite THIS hot though.)
Acrid smell is usually oil leaking. It’s going to take some looking around, above and below. It could be valve covers.
Valve covers doesn’t really make sense as both cars are doing it and the C280 smell is coming from the back of the car? Also, we have had issues with burning oil before, and this is DEFINITELY not the same smell.
2 cars with 2 different leaks. Had a tranny leak on the exhaust causing somke and smell. Valve cover leaking on exhaust, really stinky. Had broken hose leaking on intake, sweet. The only way to find it is crawl around with a light looking foe residue.
+1 to @knfenimore, you have to crawl around looking for something out of place. Since the smell is in both cars check for rodents building a nest in the car. They may be dragging the same plastic or other stuff into the cars, placing it near something that gets hot.
Do these cars have volt meters in their instrument panels? If so what is the voltage when driving at cruising speed on a hot day? The bulging battery and acrid smell are indications of over charging and although the shop tested the alternator there may be a significant difference in the voltage when operating at highway speeds and at fast idle in the shop. Also, when the battery is in the floor or trunk there is a vent/overflow tube connecting the battery case to a vent under the car and it would be difficult to determine the location of the vent from the location of the odor.
This is a wild guess, but I wonder if you may have run over a plastic garbage bag and it got melted onto the muffler or catalytic converter. It would be a strange coincidence that the problem would have happened with both cars unless the bag was somehow on the garage floor and both cars ran over it.
Are you new to living and driving in Las Vegas in the summer OP? Is this your first summer there?
The reason I ask, I’ve noticed an odor similar to what you describe in cars before when the interior of the car gets really hot or the plastic portions get exposed to direct sunlight. I’ve never noticed this here in San Jose, but I did when I lived in western Colorado, where it would get quite hot sometimes in the summer. And low humidity, considerably lower than here in San Jose. So, it might be exactly what you said it was, the odor of plastic melting. B/c portions of the plastic interior are indeed melting from the Las Vegas heat.
Have you asked your neighbors about this? Maybe this is common knowledge there.
The odor is from the fuel tank venting through the charcoal canister. The charcoal absorbs most of the hydrocarbons, the expelled vapors sometimes smell like melting plastic. Fuel odors are a common complaint in the summer from people who drive a lot in city traffic and excessive idling can contribute to this problem.
On your Camry the charcoal canister is located on the drivers side of the engine compartment, below the brake master cylinder.
Newer vehicles with returnless fuel systems normally don’t have this problem but your Camry doesn’t have a returnless fuel system. On your Camry the fuel is pumped to the fuel rail on the engine where it picks up heat, is pumped past a pressure regulator then back to the fuel tank. After an hour of city driving the fuel tank is 140F + and producing a lot of vapor.
@Nevada_545 … interesting comment. And it makes a lot of sense. There’s a canister maintenance procedure in the shop manual for my Corolla. Involves removing the canister and blowing compressed air in one port and out the other, in 2 or 3 different combinations. I wonder if something like that would work for the OP? Or is this just unavoidable, b/c when the canister vents – given its location – it is close enough to the HVAC air intakes under the windshield to get sucked into the passenger compartment by the AC? What if the OP used the AC only in recirculate mode? Do you think that would help?