Parts, accessories and the Federal Excise Tax

We are a truck body distributor that sells new taxable bodies at retail. We do not sell used bodies. When we sell a new body, we also often sell and install one or more body parts or accessories. In addition, some of our customers request that we install body parts or accessories on their used bodies. The used bodies, when new, would have been taxable under Internal Revenue Code Section 4051. Does the taxability of a part or accessory depend on whether it is installed on our new bodies or on a customer-owned, used body?