I present you the Car Bomber Mod!
Install the mod like any other ASI file - make sure you have an ASI loader like Alice or XLiveLess installed, and then drag Car Bomb.asi into your root GTA IV folder.
This mod allows you to use an unlimited number of Packie's car bombs without having to call him up
To set a bomb walk up behind a car and press F3. You can set bombs on multiple cars before detonation. When you're ready to detonate, press Shift + F3 and watch you car bombs go up in flames! Repeat process for more fun.