Which program did you use to install the car mods?
I used GTA IV Vehicles Mod Manager
In this case there should not be problem. Only you have to make sure that every vehicle has two files and that "Rebuilt Vehicles.img at exit" is checked.
So this error is very strange.
I used OpenIV or SparkIV
In this case you did it manually. Make sure no car has three or more files in the image "vehicles.img" (example: banshee.wtd, banshee.wdd, banshee_bak.wtd and so on...)
Did you installed some buildings/map mods, too?
If yes, make sure no double-file exist (no *_bak.*, *_orig.* and so on...)
