49 C.F.R. § 393.17
A combination of motor vehicles engaged in driveaway-towaway operation must be equipped with operative lamps and reflectors conforming to the rules in this section.
(a) The towing vehicle must be equipped as follows:
(b) Except as provided in paragraph (c) of this section, the rearmost towed vehicle of the combination (including the towed vehicles of a tow-bar combination, the towed vehicle of a single saddle-mount combination, and the rearmost towed vehicle of a double or triple saddle-mount combination) or, in the case of a vehicle full-mounted on a saddle-mount vehicle, either the full-mounted vehicle or the rearmost saddle-mounted vehicle must be equipped as follows:
(c) If the towed vehicle in a combination is a mobile structure trailer, it must be equipped in accordance with the following lighting devices. For the purposes of this part, mobile structure trailer means a trailer that has a roof and walls, is at least 10 feet wide, and can be used off road for dwelling or commercial purposes.
(1) When the vehicle is operated in accordance with the terms of a special permit prohibiting operation during the times when lighted lamps are required, it must have on the rear—
(d) An intermediate towed vehicle in a combination consisting of more than two vehicles (including the first saddle-mounted vehicle of a double saddle-mount combination and the first and second saddle-mount vehicles of a triple saddle-mount combination) must have one side-marker lamp on each side, located near the rear of the vehicle.



(49 U.S.C. 304, 1655; 49 CFR 1.48(b) and 301.60)
[40 FR 36126, Aug. 19, 1975, as amended at 47 FR 47837, Oct. 28, 1982; 70 FR 48044, Aug. 15, 2005; 77 FR 59828, Oct. 1, 2012; 80 FR 59075, Oct. 1, 2015]