GTA III Special Vehicle Guide: BP/FP Securicar
In this Video, I show how to obtain the BP/FP Securicar from the mission "Van Heist" with failing as it's the only way to get it. Also I show how to store it in Shoreside Vale Garage and then come back to Portland.
Use a Firetruck to easily fill the damage bar from the Securicar, then destroy the Securicar at the garage and repair it in after failing the mission. After the Securicar is repaired, take it to the building complex nearby in Hepburn Hights and starts reversing again the tree there. That will make the van sunk through the ground and eventually fall down. You will land on the top of the El Train tunnel, where you need to accelerate a little bit to make it stay there. Then get out and push the Securicar to the right side to line it up to the tunnel. Then get back into it and drive it to nearby the Porter Tunnel and fall down to respawn into it. Is a bit trick as you can't see much of the El Train tunnel to guide yourself, pretty much forcing you to blindly driving there. Luckily though, the distance to the Porter Tunnel is short, thus is likely you spawn on the tunnel. Once inside the tunnel, just drive to the garage in Shoreside Vale and store the Securicar there. Then return to Staunton by the tunnel and take a boat docked on Newport to return to Portland.
On Definitive Editon you can't use the method I used to transport the Securicar to the garage on Shoreside Vale, as you can't sunk through the ground to land on the train tunnel. That means you have to leave it on Portland until the Shoreside Vale is officially unlocked.