Hint: Vinegar seems to be the best overall answer, although the issue really has not been conclusively resolved. 🙁 Review the links below for more info:
(EXCERPTS FROM: http://www.thriftyfun.com/tf974245.tip.html)
RE: Cleaning Milk Stain In Car
By Harvey (Guest Post)
My granddaughter spilled some milk on the rear seat of my car. I wasn’t aware of it until a few days later because it went down the hole where the seat belts come out of and onto the mat that is between the bottom of the seat and the metal car floor. By then the smell was gross. I pulled out the rear seat and spread baking soda on the mat and let it sit overnight but it didn’t help. A co-worker of my wife informed her that she had the same problem in the past and here’s what always worked for her – Coffee Beans! Just spread coffee beans over the area and let it sit overnight. It smells great and absorbs all the sour smell. I spread coffee beans over the mat, replaced the seat on top of the beans and kept it there for three days and then vacuumed them up. It works. The car smells great. You have to use beans, not ground coffee.
Editor’s Note: Ground coffee also works well.