Why can't i see the modded cars that i add into the GTA IV GAME?!?!

[isogamer]

Okay so i use Spark IV to add the modded cars like Mazda, Carrera GT, Lambo, ETC. So now the only cars that i am able to spawn or see is the Lambo, Carrera GT, and some cop cars like the FBI Cavalcalde.

I don't see my Maza which replaces the SuperGT, and i don't see a Ferrari which is suppose to replace the Tursimo, which i can spawn using the GTA IV cheats, but when i use the Comet spawn GT i see the Carrera GT which i used Spark IV already to replace with.

Is there a certain amount of modded cars that i can add? What exactly am i doin wrong? I did all the steps right Import, Rebuild, and i am not sure if i am suppose to SAVE right after every single time, is that tha problem?