It needs a 3D object and is called in the spinner .gen file.
Overview you probably already know this, but in the .veh file for the car, there are pointers for the GEN files, that tell the game what models to draw. Example from a random vehicle:
Graphics=PORSCHE911.gen
Spinner=PORSCHE911_Spinner.gen
The "Graphics=" is for what is loaded in game, the "Spinner=" is for the vehicle selection screen, AKA the spinner.
Generally speaking, the Spinner will point to the same stuff as the Graphics, but simplified, it doesn't need the cockpit view, and some other small stuff that you don't need outside of being on track.
Te easiest way to do it is to put him in the seat in the car, because that 3d object already exists for the external view in game. You can normally just copy the driver stuff from the graphics gen to the spinner gen and it will show up, although you might need to play around with it.
To have the driver standing next to the car, you need a 3D model of the driver in that pose, with the textures from the suit mapped to it, and have it called from the spinner.gen I have no idea how to do that part, sorry.
