That is strange. A car mod normally doesn't consist of any script things, maybe there is some stuff inside I never used so far, I only use single car mods, no car packs cause they can lead to manifold malfunctions.
Using steam is also problematic cause nobody know what you really get with the download, my advice is obsolete as retail version user. But I heard much sh*t from steam. If you want to go hardcore modding, I recommend you to buy a retail version.
You can try XliveLess as scripting alternative, I had no problems with this one but with other script stuff I had. You can easily google for that and this will redirect you to several topics about that here.
I even don't know if that is working on steam cause XLiveless disables all online functions of the game which is actually a good thing for modding.
This post has been edited by NaidRaida on Wednesday, Apr 11 2012, 22:46