I did this with my kids in early June when the temp was 103 degrees. We just put a frying pan on the inside dash of the car, let it sit in there to get hot, and then cracked an egg in the pan. It did work! I would not recommend doing this in a nice car though,
because the hot pan did cause some minor discoloration on the dashboard. This was in a 14 year old car though, so I didn't really care.