Further manufacture: Lift axles and used trucks

Our company is a used truck dealership. In our town, there is a truck leasing company that from time to time sells trucks as they come off leases. We realize that used trucks are not subject to federal excise tax. However, the company often adds an axle to a vehicle prior to sale. Our understanding is that the leasing company charges FET only on the cost of the modification (12% of the axle and installation charges). We think that tax should apply to the full purchase price. Who is correct?