For controlling extras, use the "GTAIV SIMPLE NATIVE TRAINER" any version. 
If for some reason you can't select extras, change one line in the file "trainer.ini": 
VehicleComponents=1 - VehicleComponents=0

Extra1-front bumper's plate
Extra2-mini spoiler
Extra3-standart grille
Extra4
Extra5
Extra6
Extra7-modify grille
Extra8-some plastic
Extra9-mega spoiler
