I have found that if I put a leash on my ferret(with a harness) that they
can be loose in the car.  The leash makes it so you can get them if they are
trying to sneak under the seats.  The one thing you have to watch out for is
that they don't climb down by the drivers feet....  this makes heavy
breaking gruesome.  Just bring plenty of water, a warm lap and lots of
patience.
 
I had a rabbit.  I had a ferret.  Ferret made rabbit psychotic.  I got rid
of the rabbit.  Ferret smiled.  Now has sites set on the dog.  Dog is
concerned.
[Posted in FML issue 1443]