This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
There is a check valve to correct washer nozzle freeze ups (which dealers
may not acknowledge). (Jim Hoare)
In case of emergency, e.g. being stuck on the Interstate, you could add
rubbing alcohol to the solution, but this is not a good permanent solution
because it will not last. I found that using a higher quality wiper fluid
does solve the problem.