I am making a car mod trying to recreate
this vehicle(1973 El Camino from My Name is Earl). I am a noob at zmodeler and GTA IV, so sorry if this is a stupid question.
This is how it shows up in zmodeler:

Which is perfect.
The problem is, it turns half invisible when I try to use it in GTA IV, like this:

The car worked perfect before I added the materials to it, and it appears as though only the parts that I have added materials to are the invisible parts.
How can I fix this? Is there a certain way I have to add the materials? Thanks in advance.