When we were there in August, Lightening and Mater motored out from backstage in DCA to the directly across from the Golden Dreams Theater.
Between the Pin Cart and the coffee cart. A cast member (actor) came out with them to introduce them and they stopped for pictures. I assume they were there for about 20 minutes. It started at 11 and I believe they came out every hour until 4pm.
Check the Disney today when you arrive for Character greetings for when they will be out.