do this in a freeaim lobby it will be emptier
park the car along side the little turn, make sure the back wheels are on the side walk and the front on the driveway
run in garage ( i noticed that last location and saving icon thing dont matter)
get in car you want to replace, choose join race
hold RT and X for 15 seconds until the loading screen disappears
(this part is the trickiest) when you are in a lobby change the radio stations, youll notice it will freeze when the quit option appears, when you are able to change the station hit quit
(results vary) youll show up glitching under the garage. there is times when there is no textures/walls/garage and you can just run up and get in the car dont move until the glitching animation happens.
if the garage appears, wait for it to open before running, if you run too fast the garage will not open or youll freeze.