Thank you @jousefm for helping
But I am still stuck over merging STL files. I use CATIA for assembly. If you could provide STP file of car instead of STL, it would be of great help.
Here the front wing and the car are both at origin. I tried arranging this in 3D builder and using notepad editing. Theses methods did not work. Thus, if I get STP file, I can solve this in CATIA.