Unless the topper was part of the vehicle purchase it is its own new asset. Call it a toolbox, if less than $2500 inclusive of tax, shipping, professional install, and write it off immediately under de minimis rules. Otherwise despite it increasing potential trade in, it gets depreciated on a different scale than the vehicle.
However, if the tower purchase is within just a month or so of vehicle purchase, AND first year depreciation of vehicle has yet to be taken (a 2020 acquisition) then you can add the topper value to the vehicle asset value as a delayed optional equipment
