Steps:

1) Go Online and find a location that where the vehicle will be able to fit. If its a car, a street near your house will be fine or if its a plane, then Trevor's airfield should be good. Now, set your spawn location to 'Last Location'.

2) Press down on the D-Pad and switch to the character you have saved the car with. But if the car you are importing is a car that is readily found on the street, then it does not matter who you swap to. This step allows you to 100% have your online spawn location in the area you were last in.

3) Open up an 'Invite Only' session.

4) Once online is loaded and you have spawned in the location where you want the car to be, Press 'Start' and scroll to 'Microsoft Store' and wait for the store to load.

5) As soon as you see the store load (Shark cards start loading), press the 'Xbox home' button,press X and sign out.

6) Now heres where its changed for xbox users wireless users or modem/ethernet

6A) Wireless- When you sign out it will take you to the first single player mission,sign in then go to game>load game>and then load the save that your car you are wanting to put online is.

6B) Modem/ethernet- Unplug your connection now this will take you to your save that you had the car.So this is simple continue to 7.

7) Now retrieve the car you would like to import into multiplayer.

8) Drive it to the location where your player was standing in online. Make sure it is near the area of where you were last standing so the car does not de-spawn. I usually park it in the middle of the street. Do not exit the vehicle.

9) Sign into Xbox account (If you are using ethernet) and then quickly start up an 'Invite only' session.

10) The vehicle should be where you left it when you return to online.

Credit goes to Hitman1337