I am assuming to install vehicle texture packs, you use SPARK IV and open the corresponding model's WTD File and replace a texture? well sometimes I do that and it doesn't replace the entire vehicles textures. For Ex: If the download comes with one PNG file (usually 2048x2048), which textures inside do I replace..all of them or just one?

Textures can be installed with SparkIV or OpenIV. Generally models are released with 4 textures for the outside of a specific vehicle and are labeled police_sign_1, police_sign_2, police_sign_3 and police_sign_4. If you have +livery added to your models data line located in the vehicles.ide (Grand Theft Auto IV/Common/Data) file then it will spawn all 4 textures randomly. If you just want one specific texture to spawn the easiest way to do it is to replace all 4 textures with the one you want displayed. Now if you use the excellent Area Livery Mod by Sam installed then you can have say a city police patrolling one area (say police_sign_1), a Sheriff's Department texture spawning in another (police_sign_3), maybe state police in another (police_sign_3) you get the idea.