Probably one of the fuel lines from the rear of the car.  With all of that
cold air going under the car at 65mph might be closer to -20 degrees F on a
-4 degree day.
The best way to combat this problem is to follow your manual's suggestions
for adding kerosene (or even gasoline) to your diesel.  There should be a
chart giving you the proper mixture to avoid another possible break down.

Happy driving!
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242