This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
A problem on many A-bodies (Valiant/Dart group). May be solved by keeping
the cowl (that grille between the hood and windshield) free of leaves and
gunk, and by straightening out and emptying out the air conditioner
condensation drain. The black tube that carries a/c water may be seen on
the firewall (the metal between the engine and the driver). It is small and
behind other stuff. Sometimes the end of this black tube freezes to itself
and must be opened with a knife or razor.