Is a fifth wheel installed on a used chassis taxable?

We sell fifth wheels and install them on used trucks (which were taxable when new) in order to convert the trucks into tractors. Sometimes, we sell the fifth wheels uninstalled. We know that the fifth wheels are parts or accessories, but are unsure if they are always taxable under Internal Revenue Code Section 4051. Can you provide some general guidelines to help us determine when we should charge tax on the sale of the fifth wheels?