copied as referance only from the http://www.nhtsa.gov/Cars/rules/standards/conspicuity/Trlrpstr.html . 03/11/2110
If you scroll down to the illustrations you can click on a lights/location label and the table will referenace the related code-very cool.

TRAILERS: FEDERAL LIGHTING EQUIPMENT LOCATION REQUIREMENTS



IMPORTANT NOTE: Every lamp, reflex reflector, and conspicuity treatment (device) must be permanently attached in the location specified below and must comply with all applicable requirements prescribed for it by FMVSS/CMVSS 108. The face of any device on the front/rear and sides should be, respectively perpendicular and parallel to vehicle centerline, unless it is photometrically certified at installation angle. No part of the vehicle shall prevent any device from meeting its prescribed requirements unless an auxiliary device meeting all prescribed requirements is installed.

In Canada: Manufacturers and importers of vehicles must have the proper certification test records demonstrating compliance of lighting components with all prescribed requirements.

BASIC EQUIPMENT REQUIRED ON ALL TRAILERS
DESCRIPTION MANDATORY REQUIREMENTS
Area Equipment

 (SAE Lens Coding) Functional
Purpose		 Quantity Color Location Height mm (in.)
from the ground
1 Tail Lamps (T) Indicate vehicle's presence and width Minimum 2 Red On the rear - symmetrical - as far apart as practicable 380-1830 (15-72")
Stop Lamps (S) Indicate braking Minimum 2 Red On the rear - symmetrical - as far apart as practicable 380-1830 (15-72")
Rear Turn Signal Lamps ( I) Indicate direction of turn Minimum 2 Red or Yellow On the rear - symmetrical - as far apart as practicable 380-2110 (15-83")
Rear Reflex Reflectors (A) Indicate vehicle's presence and width Minimum 2 Red On the rear - symmetrical - as far apart as practicable- facing rearward 380-1530 (15-60")
2 License Plate Lamp(s) (L) Illuminates license plate Minimum 1 White On the rear - above or at the sides of license plate No requirement
3 Rear Side Marker Lamps (P2,PC* or P3, PC2*)
*photometrically certified at installation angle

Front and rear side marker lamps / side reflex reflectors indicate vehicle's presence and length
 
 Minimum 2 Red Each side at rear - as far back as practicable 380-1530 (15-60") no max. for veh. under 2.032m(80") wide
Rear Side Reflex Reflectors (A) Minimum 2 Red Each side at rear - as far back as practicable facing sideward 380-1530 (15-60")
4a Front Side Marker Lamps (P2, PC* or P3, PC2*)
* photometrically certified at installation angle		 Minimum 2 Yellow Each side at front - as far forward as practicable 380 (15") minimum
4b Front Side Reflex Reflectors (A) Minimum 2 Yellow Each side at front - as far forward as practicable facing sideward 380-1530 (15-60")


ADDITIONAL EQUIPMENT FOR TRAILERS EXCEEDING THE FOLLOWING PARAMETERS

LENGTH 9.1 m (30 ft.) OR LONGER
DESCRIPTION MANDATORY REQUIREMENTS
Area Equipment (SAE Lens Coding) Functional
Purpose		 Quantity Color Location Height mm (in.)
from the ground
5a Intermediate Side Marker Lamps (P2 or P3") Indicate presence of a long vehicle Minimum 2 Yellow Each side near center - facing sideward 380 (15") minimum
5b Intermediate Side Reflex Reflectors (A) Indicate presence of a long vehicle Minimum 2 Yellow Each side near center - facing sideward 380-1530 (15-60")

WIDTH 2.032 m (80 in.) OR WIDER
DESCRIPTION MANDATORY REQUIREMENTS
Area Equipment (SAE Lens Coding) Functional Purpose Quantity Color Location Height
6 Rear Clearance Lamps (P2, PC*or P3, PC2*)
* photometrically certified at installation angle		 Show vehicle's width -
MAY NOT be combined with tail lamps		 Minimum 2 Red At widest point - symmetrical - on the rear or near the rear - facing rearward As high as practicable - may be lower only if ID lamps are at the top
7 Rear Identification Lamps (P2 or P3") Indicate presence of a wide vehicle Exactly 3 Red On the rear - center, facing rearward - horizontally spaced 150 mm (6 in.) to 300 mm (12 in.) apart in Canada : at the top - maybe lower if door header narrower than 25 mm
in USA: as high as practicable
8 Front Clearance Lamps (P2, PC*or P3, PC2*)
* photometrically certified at installation angle		 Show vehicle's width Minimum 2 Yellow At widest point - symmetrical - as far forward as practicable - facing forward As high as practicable

WIDTH 2.032 m (80 in.) OR WIDER AND GVWR 4536 kg (10,000 lb.) OR MORE
DESCRIPTION MANDATORY REQUIREMENTS
Area Conspicuity Treatment DOT Coding Quantity Color Location Height OPTIONS
9 Rear Upper Body Marking

DOT-C,
DOT-C2,
DOT-C3, or
DOT-C4
 
 Exactly
2 pairs of 300mm long strips		 White On the rear upper corners - facing rearward At the top Reflex reflectors may not be required if they are replaced, in their required location, with conspicuity treatment
Optional in Canada

Rear lower body and side conspicuity treatment may also be solid white, solid yellow, or white and yellow.
10 Bumper Bar Marking Continuous Red/White On the rear bumper bar's horizontal element - full width - facing rearward No requirement
11 Rear Lower Body Marking Continuous Red/White see options On the rear - full width of the vehicle - facing rearward As horizontal as practicable and as close as practicable to the range of 375 to 1525 mm from the ground
12 Side Marking see location Red/White see options Each side - facing sideward - continuous, or evenly spaced over minimum of 50% of length, starts and ends as close to the front and rear or vehicle as practicable As horizontal as practicable and as close as practicable to the range of 375 to 1525 mm from the ground

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This table summarizes lighting equipment requirements contained in Federal/Canada Motor Vehicle Safety Standard 108.
For complete compliance requirements consult Title 49 - Code of Federal Regulations-Section 571.108 (USA) or Section 108 of the Motor Vehicle Safety Regulations (Canada).

State or Provincial regulations, where they may apply, have not been included.

For more information contact: