you have to rebuild the image after placing all the files. i assume you use rw analyze and gta3 img tool? Also having txd workshop. you have to have all 3 of these for a car mod to work as well as dmajic's wheelmod. That could be one of your problems.
I'm sure you know most of this though.
you also have to edit some data files which should be included in the read me with the mod you are using.
let me know if this helps.
XX-73