Things you need
Grand Theft Auto 5
A Friend
A garage with a drive animation *Works best with Eclipse Towers, Weazel Plaza, Tinsel Towers and Richards Majestic*
Set your spawn location to Last Location/Garage

Have your friend sit in the car while you attempt to store the car, otherwise it might vanish!

Park the car you want outside your garage and follow these steps
Get in the car you wish to replace
Select Quick Job, Join Race
You need to press RT and A at the exact same time
Hold them both
Let go of A when you see the online tip
Keep holding RT
Press B and leave as soon as you can!
Keep holding RT, you should spawn in the garage, just turn around and get in the car!
It will spawn you in the garage driving out! just take out any car from your garage and then put it back in!
If it spawns you in the garage and it kicks you out just walk out and back in
take out a car and put it back in!

If you try to get in the car and it kicks you back into the garage have someone else host the session.

Things that cause this to fail
You failed to press RT and A at the exact same time
*If you press A first it will spawn you in the mission, when you leave you will spawn at the mission marker and have to drive back - If you press RT first you will simply drive out of your garage*

If you would like more help send me a PM and I will help as much as I can