GTA SA: Special Merit

In the mission "555 We Tip", it is claimed by some that a player can obtain a bulletproof, fireproof, damage proof and explosion proof Merit. The method is described on the "special vehicles in gta sa" page, yet it is unclear on the "leave the car open to the garage (as the car is locked when you close it), make some fire, then enter the car while on fire to destroy it and to fail the mission" part.

Do anyone know about how? please clarify this :D