in spark iv just click "save". don't click rebuild or it will restore the archive as it originally was.
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the cars won't show up right away when you're playing.
they might not show up at all if they are high poly models (because of the amount of memory the game has to load in).
if you have a bunch of high poly mod cars, you'll start to get what is called "taxi bug".
basically, the game in it's vanilla form, will, after several car groups have passed, load taxis to "refresh" it's memory.
if the memory is overloaded, it'll do this continuously because the game thinks it's running on 2006 xbox 360 and is trying to conserve memory.
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to answer your question:
to get the car models to appear, use ikt's mod called "traffic load":
http://www.gta4-mods.com/script/trafficload-f9780traffic load spams random cars and helps prevent taxi bug. after awhile, it'll start to spam the car mods as well if not most of the time.
make sure you have a lot of vram (at least 2gb on your vid card) because it will primarily pick the cars with the largest file size first after awhile.
another way to spawn your mod cars is to use something called "car spawner":
http://www.gta4-mods.com/script/carspawner...ersion-22-f2289spawn your mod car, and after awhile you'll start to see that car appear often in the car group.
it causes taxi bug, so make sure you are using traffic load as well.