You get the Porsche when you Solve Side Quest 35 (Chippin In). But Before you can play these Side Mission, you have to finish Mission 34 (Tapeworm).
How to Solve Side Quest 35 (Chippin In)
Mission Guide /Location Map
Go to Afterlife and speak to Johnny at the entrance. After the scene with Johnny and Rogue ends, you find yourself in a hotel room in Northside. Next, you should take care of other missions until Rogue finally gets back to you.
After Rogue's call, you meet with her and Weyland at Afterlife to discuss the plan.
Now you can go to the shipyard on the north edge of Northside, where you should find a computer in the middle of a Maelstrom hideout. Here, too, different paths lead to the goal - more about this under the heading The Maelstrom Camp.
After examining the computer and reading the message regarding the Ebunike, go to the long staircase that leads up to the ship. Note that you will have to deal with two guards on the train station. There are also other enemies on deck. When you get to the other end of the ship, a fight against Grayson will inevitably start. You should always keep moving and use the many boxes in the area as cover. After your victory, Grayson's fate is in your hands. To spare him (Your lucky day) has the advantage that he offers you a key that gives you access to a special reward. To do this, use the control panel at the top of the ladder nearby to lower the large container that is hanging on the crane. Inside is Johnny's old Porsche. This is your only chance to get it; so you shouldn't miss it.
The Maelstrom Camp
Follow Rogue on the edge of the camp, then she will lead you to a stack of containers (watch out for the two gang members patrolling nearby). After you have opened the container doors, you get to the hiding place. If you crouch along the wall to the left, you will only meet two guards on the way to the building with the data terminal: one in the corner in front of the small building and one directly behind it. Both are on their own and easy to neutralize or circumvent. If you enter the building with the data terminal through the door on the ground floor, you will face another Maelstrom, but there are plenty of boxes to hide. The computer is waiting for you on the other side of the glass door.