Thank you, that makes more sense.
I would imagine if a horse was ridden in the rain, or was sweating profusely, you'd want to brush out his mane.
Put away wet, the horse will look ragged and tangled tomorrow.
While this does not translate literally to cars, it must be used to describe something in ratty appearance.
http://www.wisegeek.com/what-does-rode-hard-and-put-away-wet-mean.htm