Puzzling graphics question (no replies)

I'm trying to change some of the cars in a pre-existing mod, the 1991 mod to be specific. When I select any of the cars I've installed or the in-game camera is on them, they appear as intended. However, if they are not selected or the in-game camera is not on them they revert back to the original livery of the car before any changes were made.

I looked in the FAQ section but could not see anything that directly addressed this problem. As a noob I'm sure it's something silly I'm not doing, but any suggestions or pointers would be greatly appreciated.

Thanks.