Chronic licking

Question
I have a 5 y.o. spayed female, flat-coated retriever. She is very bright, easily trained, loving, ball retrieving fanatic, generally obedient. She does suffer from nocturnal enuresis, which is easily controlled by Proin. She is otherwise very healthy.
Since the moment I adopted her from the flat-coat rescue (family surrendered) 2.5 years ago, she has licked the windows in the car. This is the only non-human object she licks to the point that the windows are frothy. I have resorted to putting her in a crate with a ball to chew on while she rides in the car. She is the first one in the car to go for a ride. She'll even lay in the back of the car in the driveway while I do yard work. What I am trying to say is that she does not seem to have a fear of the car.

In the house there is no licking, no apparent anxiety. However, she likes to lick me constantly if my face or skin is anywhere near her tongue. When she and I play and I am on the floor with her she will also resort to 'nibbling' at my scalp. Much like one dog does to another when grooming each other.

Why the licking?

Answer
You appear to have solved the car window licking by crating the dog, which is safer for the dog.  She may be licking your face and hands as an appeasement gesture, an affectionate display, or because she likes the taste.  This is really a non problem.  Dogs are individuals and not every single behavior can be explained.