Maybe you forget to save each time .. just try this
- open up spark IV, navigate to pc/models/cdimages/vehicles (.dat I believe?)
- Open up the image, import the downloaded vehicle (usually a .wft AND a .wtd)
- Press save
- Exit Spark IV
I'd suggest using some mod for spawning vehicles, 'simple native trainer' is the easiest to use program that can do pretty much everything.
Oh patch 188.8.131.52. is perfectly moddable yes, I use it also