Are you installing the same car each time? Copy the vehicles.img from your backup folder (pc/models/cdimages) and place it back (or just open up the backup img and export the files you have replaced.
Place back your backup and try again with an other vehicle. Are you sure you're importing all the files that come with the package?
Oh I just remembered something, do you load a saved game? Do you have parked cars out front? If so you should try starting a new game first. I remember that causing an error with me too because I had that car parked out front.