The Right Spark Plug in the Right Place (1945)

A motorist whose engine is running poorly pulls into a garage, where the mechanic, rather than simply replacing his spark plugs, goes into extensive detail on the various kinds of spark plugs available (heat ranges, etc). The motorist and mechanic are seen (except for the opening and closing shot of the motorist driving) as shadows on the wall of the garage. Goofy and Pluto appear in brief cameos to show us an unconventional model of automobile.