Here is a glitch for you to modify cars to your liking in single player, and then bring them to multiplayer.
1. Get a car and modify it. Go to Michael's safehouse and park the car wherever you want.
3.Go to Online.
4.Hover over Money & Services
5. Start Lag Switch
6. Click Money & Services Once
7. Click Start & Go Open Up Xbox/PS3 Gamestore
8. Click Start Again, & Leave GTA 5 Online
9. Stop Lagswitch once Store Loads Up
10. Exit store, start lag switch once stroy player is almost loaded up.
11. Run To The Garage and Hop in the Modded Car.
12. Stop Lag Switch As Soon As you Get in the Car!
*I suggest putting Lag Strength on Insane
Using the editor: