Debate Settled–Illegal For Unsecured Animal in Back of Truck
Straight from the Benton County Sheriff, it's illegal for a dog or animal to be riding in the back bed (regular, flat, or otherwise) of a vehicle or truck unless it has a suitable cage, enclosure or harness.
Sheriff's deputies say they're seeing an increase in the number of animals (especially dogs) running around unsecured in pickup beds. Many might not be aware, but there is a Washington state law about this. (RCW.46.61.660 for those who don't believe it).
It requires an animal to have either a secure harness, cage, or other enclosed compartment to ride in, if being transported in an uncovered open vehicle bed, traditional truck or otherwise. Would seem to apply to convertible cars too, although they're almost unheard of now.
So if an officer sees your doggie or other animal leaping and bounding without any 'security' it could cost you $136.