I have found that on the really nasty areas that regular car wash detergent just can't seem to work on, some of LA's Totally Awesome cleaner diluted either by itself or mixed in with the car wash detergent works well to clean those areas. It can be purchased at the Dollar type stores. It will remove the wax, so only use where really needed.
https://www.amazon.com/LAS-PURPOSE-C.../dp/B0017KT3IM