Hello, and welcome to FF,
most Brits know what a West Highland Terrier is ,
look up "Hamish McBeth" and his sidekick "Robbie"
I've not owned a 500 ,but rented a few,
in my mind the dog may be safer/ comfier in the Boot of the 500 with the parcel shelf removed, and a dog grille from the seat back to the cars ceiling / headliner,
then you still have the conventional 4 seats available,
but obviously you will not have the use of the boot...,
of course with a "wee dog" it wont be in your sight behind the seat back.. so a dog restraint.. joined to a rear seat belt may be more sociable..!!,
as an aside - people who go into the boot multiple times a day, have reported wear the the wiring, through the opening + closing process,
look / ask in the 500 section, as the 500 has been around for a while now,
they will probably know all the relevant pro's / con's,
with dogs AND Sat Nav's,
enjoy your new car..!!,
Charlie