In this Video, I show how to obtain the Unique Corpse Manana from the mission "Dead Skunk in the Trunk" with failing the mission 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.
Before entering the Manana, destroy the Forelli's Sentinels so they don't chase you. You can easily do it with Grenades like I shown in the video. After that you can do two things, either destroy the Manana in the garage and repair it, or ditch it in the water. Both ways let you keep the car after failing the mission. I gone for ditching it as super easy and doesn't take you alot of health. Once you have the car, take it to the building complex in Hepburn Hights near by Claude's garage. We gone to that place in the BP/FP Securicar video to sunk it through the ground, and we be doing that with the Manana too. Though instead reversing into the tree, we gonna reverse in between the corner of the building and the wall. After you sunk it through the ground, you will land on the top of the El Train tunnel. Then you have to drive and fall close to the Porter Tunnel to spawn into it. Unlike on Securicar, the Manana first-person view doesn't have the train tunnel disappearing. So you be able to guide yourself this time on the way to the Porter Tunnel. Once you spawn inside the tunnel, store the Manana in Shoreside Vale Garage. Then return to Staunton through the tunnel and take a boat from Newport back to Portland.
Don't convert this vehicle into Damage Proof as you wouldn't be able to damage the trunk and see the body if do that, thus losing it's speciality.
This particular Manana is called "Corpse" inside the game files. Because of it is a separate vehicle from normal Manana, and a Unique Vehicle as well.