Dear Mr. [ ]:
This responds to your letter of October 7, 2003, regarding certain trailer lighting requirements. You intend to install a set of two lamps at the top rear corners of the trailer. The outer-most lamp would have a dual function of a clearance lamp, and an "auxiliary" turn signal lamp. The horizontally adjacent (inboard) lamp would function as an "auxiliary" stop lamp and, possibly, as a second clearance lamp.  You ask whether such a lamp configuration is permissible under Federal Motor Vehicle Safety Standard (FMVSS) No. 108. Further, you are aware that FMVSS No. 108 limits the number of rear identification lamps to three. You ask at what horizontal distance from these identification lamps would supplemental lamps be deemed "auxiliary" and not a part of this identification lamp cluster.
Before I address your questions, I note that you requested confidential treatment for the identity of your company and for yourself. I agree to keep your name and the name of your company confidential. All information in bold brackets [ ] will be kept confidential from the public. Your incoming interpretation request will be redacted before being made publicly available.
By way of background, the National Highway Traffic Safety Administration (NHTSA) does not provide approvals of motor vehicles or motor vehicle equipment. Under 49 U.S.C. Chapter 301, manufacturers are required to certify that their vehicles and equipment meet applicable requirements.
With respect to your first question, the dual function "auxiliary" lamps described in you letter are permissible under FMVSS No. 108 with certain limitations. First, Table II of FMVSS No. 108 requires that the rear clearance lamps be red. Accordingly, the outer-most clearance lamp (that will also function as a turn signal lamp) must be red. Second, the stop lamp that is adjacent to the clearance/turn signal lamp, cannot also function as an additional clearance lamp, because Table II of FMVSS No. 108 requires that clearance lamps indicate the overall width of the vehicle. Accordingly, only the outer-most turn signal lamp can function as a clearance lamp, because it is the outermost lamp that indicates the overall width of the vehicle. We note, however, that it would be permissible for the adjacent inboard lamp to have a dual function of an auxiliary stop lamp and an auxiliary tail lamp.
In response to your second question, Table II of FMVSS No. 108 provides precise guidance for the location of the five lamps that are required at the top rear of the trailer. Specifically, Table II requires that the three identification lamps be located as close as practicable to the top and center of the trailer, spaced between 6 and 12 inches apart. Similarly, Table II requires that the two clearance lamps be located as close as practicable to the top of the trailer and as close to the edges as would be necessary to indicate the overall width of the trailer.
While the location of auxiliary lighting on the top rear of the trailer is not regulated by FMVSS No. 108, S5.1.3 of the standard does prohibit installation of lamps that would impair the effectiveness of required lighting. Therefore, your auxiliary lamps must be located far enough away from the three identification lamps, that they do not impair their effectiveness. In a recent interpretation letter, the agency stated that the function of this three-lamp identification cluster is to indicate the presence of a large vehicle in the roadway. The agency has long maintained that highway traffic safety is enhanced by the familiarity of drivers with established lighting schemes, which facilitates their ability to instantly recognize the meaning the lamps convey and respond accordingly. Therefore, any auxiliary lighting on the top rear of the trailer must be located such that it would not interfere or be confused with the three-lamp identification cluster.
After studying your diagrams, it appears that your auxiliary lamps would not impair the effectiveness of the required three-lamp identification cluster. The three-lamp identification cluster is located at the top center of the trailer. By contrast, your two auxiliary lights are located toward the edges of the trailer, adjacent to the clearance lamps. Thus, your rear lighting configuration would be permissible under FMVSS No. 108. 
I hope you find this information helpful. If you need further assistance, please contact George Feygin of my staff at this address or at (202) 366-2992.
 You have enclosed drawings and a description of two versions of a rear lighting configuration. In one of the two versions, the lamps adjacent to the outer-most turn signal supplemental lamp also functions as a second clearance lamp.
 You may also want to review our Interpretive Rule on the proper location of identification lamps and clearance lamps, which NHTSA published on April 5, 1999 (64 FR 16358). A copy is attached.