If you take Mucinex during the day to move the mucus out
you might have less at night. Mucinex comes in a decongestant added formula too if you prefer. Drinking more water during the day helps too.

I take mine at night, too. Antihistamines are too drying and may provoke MORE congestion over time.
Allegra is okay, Claritin is rather useless and ineffective IMO.

Foods containing histamine or histamine releasers, may provoke more nasal symptoms. I have a histamine thread in the PN subforum.

I personally do not like steroids, esp nasal. They are implicated in causing glaucoma.

Try to not have the car vents blowing on you when you drive.
And spray them with disinfectant periodically. Lots of spores of fungus come out of those vents. In spring there is pollen galore also. There is a filter in newer cars, behind the glove box, that can be replaced as well.

This is a sample video... I would assume each car is different in minor ways:
http://www.youtube.com/watch?v=zGXyu3g403E

If you decide to use a steroid, and notice an increase of symptoms, I would suspect a fungus infection. This is pretty common, but needs treatment with antifungals.
Any infection present may be increased or fueled by the steroid, as steroids suppress immune response in the tissue.