From my experience it takes 2 normal grenades to make a vehicle blow up.
When an enemy is taking cover behind a vehicle use a grenade/launcher. The result will be the vehicle starting to catch fire. Now use your second grenade to destroy the vehicle.
If an enemy has full health you may need to weaken it so that the car explosion can do enough damage to get the kill. In this case simply position your first explosive so that it covers both the enemy and the car.