The UPS truck replaces the Humane one. That I dont think spawns naturally. The Fedex truck repaces the default mail truck.
Hmm, strange. I will look into this to find out why they not spawning. Are they not spawning at all or with default textures. They have the textures spread between a crap ton of files.
@KubiiQ - I am unsure about that. This never happened for me.
@Toad - Make sure you "Rebuild" your rpf file. While still in edit mode, after to added the new textures, go to File -> "Rebuild"