My husband and I travel quite frequently with our ferrets and can suggest
the following.  Ensure that the air is sufficient in the vehicle (i.e. not
to stuffy and not to cold).  They will sleep like 99% of the time while in
the car.  If you are able to place the cage in the vehicle do so - it will
help a lot.  We find that if we have the cage they spend their time in there
if we don't the tend to locate any open bag and Stake their claim:)
 
When you stop get them out of the car (especially if you are traveling
through a place that is really hot or really cold) - never leave them in the
car alone for an extended period of time (you know as well as I that there
is always the possibility of them DRIVING off without YOU!
 
Good luck and happy travels,
dawnie, bob and the three ferreteers
[Posted in FML issue 1443]