1 - open SparkIV
2 - choose the game, GTAIV or EFLC
3 - wait, and go to the vehicles.img in /pc/models/cdimages
4 - click Import, select the correct wtd and wft files, wait a bit
5 - click Save, wait, click Rebuild, wait again
6 - close the vehicles.img window, click Save in main SparkIV windows, wait a bit, and close
Sometimes it's important to wait between each step, because CPU needs to load the stuff.