FET and the six-month rule on parts and accessories

​​​​​​​A customer recently purchased a new truck chassis, and the invoice for the truck included FET. They now want our company to install an additional part on the truck chassis. (The part would have been taxable if it was included on the truck when the truck was sold.) Since the customer already paid tax on the truck, is there any tax risk for our company under Internal Revenue Code Section 4051?