That depends on the mod.
They are usually located in the MAS files, you can open them with this tool:
[
www.rfactor.net]
To know what are the MAS files where the steering wheels are located you have to open the GEN file of the car (with Notepad). You'll find something like that:
MASFile=xxxxx (where xxxx is the filename)
So that are the MAS files where the steering wheels can be.
Also, I think you know that, but in rFactor steering wheels are 3D, unlike in GP4, where steering wheels are 2D. So if you want to change the steering wheel completely you have to edit the 3D mesh too.