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See Hucho, page 183, Fig. 4.74 for a picture of an Opel Calibra (said to be about the most aero-efficient production car of recent times) and note the ~60 degree outboard deflection angle of the smoke trail, which then impacts the front tire for even more drag. If this is not interference drag, I dunno what is.
Also see the various cites and illustrations in Hoerner's "Fluid Dynamic Drag," the aerodynamicist's Bible. When one body is too close to another body, the flow from each tangles with the other, for total drag greater than the sum of the parts. Hoerner cites studies of biplane wing placement and interference, bombs/drop tanks hanging from fuselages and wings, etc.. Basically, this says you need to keep such bodies adequately separated, so the airflow has a smooth and sufficiently wide gap to go.
Bruce Carmichael is one of the best modern aircraft aerodynamicists, and has published in Sport Aviation and Soaring magazines for decades, in addition to his own books and papers. He tells of the Douglas DC-3, which had a streamlined fuse and a streamlined wing. Putting the two together made a ~63% increase in drag, due to interference.
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