Federal law requires a backup lamp
"
e) Backup lamps must be mounted
on the rear so that the optical center
of at least one lamp is visible from any
eye point elevation from at least 1828
mm (6 ft) to 610 mm (2 ft) above the
horizontal plane on which the vehicle
is standing; and from any position in
the area, rearward of a vertical plane
perpendicular to the longitudinal axis
of the vehicle, 914 mm (3 ft) to the rear
of the vehicle and extending 914 mm (3
ft) beyond each side of the vehicle"
"Backup lamp .................... 1 white ......................................................... None ......................... J593c, February 1968."
Yes, they are required under various state codes too, I have had vehicles tested for working reverse lamps when checking for VINs and applying for titles under GA's R22 forms.
Georgia Title 40. Motor Vehicles and Traffic Section 40-8-26. Read the code on FindLaw
codes.findlaw.com
"(d) All lenses on brake lights and signal devices shall be maintained in good repair and shall meet manufacturers' specifications."
It is considered a signal device (that you are intending to back up).
I put in LED bulbs off of E-Bay on my trucks and did not have to add any resistor. The 194 LED worked fine in my Bronco 2 dash too. The only problem I had was with the bed lamp in the F-150, I guess the 194 socket only has leads on one side and I have to flip the 194 LED around.
$0.02