What's illegal about this load?

We have a winner. Steady burning Amber light on the rear is not legal.
Amber stop and turn signals are ok as long as they don't light as a tail light.

No. §393.11 requires that lighting devices on Commercial Motor Vehicle CMVs placed in operation after March 7, 1989, meet the requirements of FMVSS No. 108 in effect at the time of manufacture. The National Highway Traffic Safety Administration (NHTSA) has issued interpretations which indicate that the use of amber tail lamps impairs the effectiveness of the required lighting equipment and as such is prohibited by Federal Motor Vehicle Safety Standards (developed and issued by the NationalHighway Traffic Safety Administration) (FMVSS) No. 108 (S5.1.3). Since National Highway Traffic Safety Administration (NHTSA) does not allow vehicle manufacturers to install amber tail lamps, the Federal Highway Administration (FHWA) has concluded that the use of amber tail lamps on vehicles placed in operation after March 7, 1989, is prohibited by §393.11.