According to the United States "Flag Code", Title 4,7,(b);

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"The flag should not be draped over the hood, top, sides, or back of a vehicle or of a railroad train or a boat. When the flag is displayed on a motorcar, the staff should be fixed firmly to the chassis or clamped to the right fender."

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There are a number of additional "violations" possible when displayed on a vehicle though.

Section 6,(a); "It is the universal custom to display the flag only from sunrise to sunset on buildings and on stationary flagstaffs in the open. However, when a patriotic effect is desired, the flag may be displayed twenty-four hours a day if properly illuminated during the hours of darkness."

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Section 8,(b); "The flag should never touch anything beneath it, such as the ground, the floor, water, or merchandise."

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Section 8,(e);  "The flag should never be fastened, displayed, used, or stored in such a manner as to permit it to be easily torn, soiled, or damaged in any way."

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Section 8,(i); "The flag should never be used for advertising purposes in any manner whatsoever. It should not be embroidered on such articles as cushions or handkerchiefs and the like, printed or otherwise impressed on paper napkins or boxes or anything that is designed for temporary use and discard. Advertising signs should not be fastened to a staff or halyard from which the flag is flown."

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Full text: "The United States Flag: Federal Law Relating to Display and Associated Questions"