Soapy water with a wash mitt has always worked for me.
Then try a different soap. There is nothing magical about the rear of the car that prevents dirt from being washed away. The only other possibility is it's not road dirt on your car but is some sort of petroleum grease, oil, tar, etc. For that you may need mineral spirits.Thanks, very helpful, I will try these. I have only used a wool mitt and soapy water and rinse. That's fine for everywhere except the back of the car.
I have a Volt, so my question is, what's brake dust? hahahait was amazing to simply rinse brake dust off.