As you have surmised, the actual flag code does not specify front or rear of vehicle and other's assertations that it has to be on the front are simply based on opinion, not fact or law. I invite you to read the code yourself and make your own judgement based upon what is actually written there, not on what others tell you.
United States Code, Title 36, Chapter 10
From the above highlighted flag code...
§175. Position and manner of display
The flag, when carried in a procession with another flag or flags, should be either on the marching right; that is, the flag's own right, or, if there is a line of other flags, in front of the center of that line.
- (a) The flag should not be displayed on a float in a parade except from a staff, or as provided in subsection (i) of this section.
- (b) 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 shall be fixed firmly to the chassis or clamped to the right fender.
From Army regulation AR840-10
2–4. Position and manner of display
a. Ceremonies and parades.
(1) The flag of the United States will be carried on all ceremonial
occasions when two or more companies or an appropriate honor
guard participates. It is always displayed in the position of honor.
(2) When the flag of the United States is carried in a procession
with other flags, the place of the flag of the United States is on the
marching right; or, if there is a line of other flags, in front of the
center of that line. (See fig 2-1).
(3) The flag should never be carried flat or horizontally but
always aloft and free.
(4)
When the flag of the United States is displayed from a vehicle,
the staff of the flag will be clamped firmly to the right front
fender.