I agree with others on here who have advised against leaving a dog unattended in a car, not least of all for the possibility of being stolen (cockers are VERY high on the list of stolen dogs) but also because of the huge risk of them becoming overheated and dying as a result, in as little as 20 minutes.
It really doesn't need to be a hot sunny day and even in the shade, the temperature in the car can rise rapidly. It is a metal container which conducts heat well.
I've sat in a car, in the shade, and had to get out because it was so hot and stuffy and that wasn't in particularly hot weather. A dog inside a closed vehicle would stand no chance.
Please be careful.