26 USC 4003 - Special rules

(a) Separate purchase of vehicle and parts and accessories therefor 
Under regulations prescribed by the Secretary
(1) In general 
Except as provided in paragraph (2), if
(A) the owner, lessee, or operator of any passenger vehicle installs (or causes to be installed) any part or accessory (other than property described in section 4001 (a)(2)(B)) on such vehicle, and
(B) such installation is not later than the date 6 months after the date the vehicle was 1st placed in service,

then there is hereby imposed on such installation a tax equal to 10 percent of the price of such part or accessory and its installation.

(2) Limitation 
The tax imposed by paragraph (1) on the installation of any part or accessory shall not exceed 10 percent of the excess (if any) of
(A) the sum of
(i) the price of such part or accessory and its installation,
(ii) the aggregate price of the parts and accessories (and their installation) installed before such part or accessory, plus
(iii) the price for which the passenger vehicle was sold, over
(B) the appropriate applicable amount as determined under section 4001 (a)(2).
(3) Exceptions 
Paragraph (1) shall not apply if
(A) the part or accessory installed is a replacement part or accessory,
(B) the part or accessory is installed to enable or assist an individual with a disability to operate the vehicle, or to enter or exit the vehicle, by compensating for the effect of such disability, or
(C) the aggregate price of the parts and accessories (and their installation) described in paragraph (1) with respect to the vehicle does not exceed $1,000 (or such other amount or amounts as the Secretary may by regulation prescribe).

The price of any part or accessory (and its installation) to which paragraph (1) does not apply by reason of this paragraph shall not be taken into account under paragraph (2)(A).

(4) Installers secondarily liable for tax 
The owners of the trade or business installing the parts or accessories shall be secondarily liable for the tax imposed by this subsection.
(b) Imposition of tax on sales, etc., within 2 years of vehicles purchased tax-free 

(1) In general 
(A) no tax was imposed under this subchapter on the 1st retail sale of any passenger vehicle by reason of its exempt use, and
(B) within 2 years after the date of such 1st retail sale, such vehicle is resold by the purchaser or such purchaser makes a substantial nonexempt use of such vehicle,

then such sale or use of such vehicle by such purchaser shall be treated as the 1st retail sale of such vehicle for a price equal to its fair market value at the time of such sale or use.

(2) Exempt use 
For purposes of this subsection, the term exempt use means any use of a vehicle if the 1st retail sale of such vehicle is not taxable under this subchapter by reason of such use.
(c) Parts and accessories sold with taxable passenger vehicle 
Parts and accessories sold on, in connection with, or with the sale of any passenger vehicle shall be treated as part of the vehicle.
(d) Partial payments, etc. 
In the case of a contract, sale, or arrangement described in paragraph (2), (3), or (4) of section 4216 (c), rules similar to the rules of section 4217 (e)(2) shall apply for purposes of this subchapter.