When Fred and Barney are on in Fred's car heading back to Fred's house before Fred kicks out the student boarders, the driver's seat is on the right side of the car. Normally, the driver's seat is on the left side of the car.
Near the end of the first season episode "Rooms for Rent", during the scene when Betty and Wilma say goodbye to the student boarders, Wilma is shown speaking with Betty's voice.