A sale of an automobile truck, truck trailer or semitrailer, shall be considered to be a sale of a chassis and of a body enumerated in this paragraph (a)(1).
For purposes of paragraph (c)(3)(i) of this section, a part is a replacement part, regardless of when it is ordered, if its use with a vehicle is as a replacement for a part on such vehicle. For purposes of paragraph (c)(3)(ii) of this section, the term aggregate price of parts and accessories (and their installation) refers to all purchases and installation charges, not including replacement parts and accessories, made with respect to a vehicle within the 6 month period provided for in paragraph (c)(1) of this section. If the aggregate price of parts and accessories (and their installation) during the 6 month period exceeds $200, the tax imposed under section 4051(b)(1) and paragraph (c)(1) of this section shall apply to the cost of all parts and accessories (and their installation) during such period. For example, a vehicle is purchased and placed in service on July 1, 1983. On August 1, 1983, the owner purchases and has installed parts and accessories at a cost of $150. On September 1, 1983, the owner purchases and has installed parts and accessories at a cost of $300. On September 1, 1983 a tax of $54 will be imposed (12 percent * $450). Any costs of additional parts and accessories installed with respect to the vehicle before January 1, 1984 (and the cost of installation) will also be subject to the 12 percent tax.
An incomplete chassis cab which is not equipped with any of the devices set forth in paragraphs (e)(1)(ii) (A) through (E) of this section shall be treated as a truck if the purchaser certifies in writing that the vehicle will not be equipped for use as a tractor.
26 C.F.R. §145.4051-1