In town a sticker might work for you or against you. Depends on how aggressive the animal control people are.
As for busybodies worried about my dog I have a sign on my dash:
Problem with my dog?
Please call XXX-XXX-XXXX
Plus I have water for him visible, a visible thermometer and a fan running. Most animal welfare ordinances require trying to contact the owner before breaking in to rescue a distressed animal. For places where it is illegal to leave a dog unattended in a vehicle (Tucson, AZ being one), I simply don't go there.
When out in the boondocks, a recording of a vicious dog triggered by a motion sensor, along with visible evidence of a large dog (big water bowl, hefty rope or chain, chewed up large bone or rawhide) would be a deterrent if one cannot see inside the vehicle.
-- Spiff