i was having the same problem about few months ago after helping my friend to move some boxes. the xiu keong is very small size as u mentioned. I had have tried many ways, here are some of my advise.

1. open windows even for days doesnt work, and u will bring more insect into your car.

2. park under hot sun doesnt work, because xiu keong can still survive even under extremely high temperature. i parked my car under hot sun for 10 days, but they are still there with more kids.

3. sheldtox and resect doesnt work. i took out all the car seat and carpet, armrest, everything, and spray, then left my car there for 3 days. this will kill most of the xiu keong, but u cannt kill the eggs. they will come back in few days.

4. i found most of them are hiding under the car seat, especially back seat. if ur car is wira, it is very easy to take the seat out and check.

5. finally, i found the way to eliminate them. go to tesco or giant to buy the insect trap and also the xiu keong poison. the insect trap is the small paper house with super glue. u can place some food in the middle to trap the xiu keong u can put 2-3 of this in your car. the xiu keong poison is the best. i bought the resect brand one. it is black color plastic cover with a small pieces poison inside. u need 3 of this in your car. i got about 10-20 xiu keung every night. after a week, they are gone. but i still leave the insect poison in my car until now just in case.