If you have access to a introductory Heat Transfer textbook, you should be able to find out more information about this topic. Most hx books contain at least baisc info about it and may direct you to further reading. Your local library may not have them, but should be able to order them on inter-library loan. Heated surfaces would have some positive effect but on ordinary vehicles the difference wouldn't be enough to notice because the points of separation are very distinctly due to geometry. ie behind the rear-view windows and cabin.
I'm sure if results were to be taken in the lab, positive results could be recorded for shapes that are more aerodynamic which do not regularly experience flow separation but my guess is that positive results would be obtained only at higher temps than would occur from solar heating.
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