Watch this first to see exactly where stand/park for the steps
Step 1) Set your spawn location to "Last Location" if it isn't already set to it for this glitch to work. Also, make sure you have no personal vehicles out. Stand outside LSC, then pause, go to online, then simply go to "Leave GTA Online"
Step 2) Once you're in single player, go to the car you desire. You don't even need to be in it, you just need to be able to see it. So I'll use Franklin's Buffalo as an example like the guy in the video did. Stand next to the Buffalo, then pause, go to "Online," then start an invite only session.
Step 3) Once you're in your Invite Only Session, you should spawn outside/across the street from LSC. Now, take a random car and drive it into LSC. Now, pause and go to the Playstation Store. As the camera zooms out above the city, press the Playstation button on your controller, go to "PSN," hit triangle over "Account Management," then hit "Sign Out." This will disconnect you from PSN resulting in an error message once the GTAV Playstation Store loads up, which will then bring you back to single player.
Step 4) Once in single player, go back to Franklin's Buffalo, then bring it to LSC. Drive up to the LSC garage door. Once it opens, simply park the car halfway through LSC, so if the garage door were to go down, the car would stop it from closing.
Step 5) Stay in the Buffalo while doing this next part. Sign back into PSN, then pause, go to "Online," and start an Invite Only Session.
Step 6) You should spawn in once again nearby LSC. Go to the LSC garage door and voila; Franklin's Buffalo is there, parked right under the garage door.