Okay if you were like me and spawned in your house/apartment/property you know you would lose some of your modded vehicles in your garage. Even if you didn't enter it. However, if you dashboard at the time or when you leave before it saves your cars will still be there. To prevent your cars from being deleted do the following:

1.) Start back up GTA Story mode
2.) Have someone invite you to a mission and start it up
3.) During that mission call up your mechanic and have him bring your modded car
4.) Destroy your modded car and repeat for all modded cars
5.) Switch spawn location to "Last Location"
5.) Finish end mission

- What this does is allow you to spawn inside the map and not in your property, the last location of the mission you just did. It also allows you to enter your garage without deleting your vehicles because your cars aren't in there anymore. This then allows you to still keep your modded cars whenever you want them simply by calling Mors Mutual Insurance and then having your mechanic bring your car to you. When your done with that vehicle just destroy it before you get off this way it sits in the Mors Mutual Insurance lot.

- Hopefully you didn't start your game up and have it save before you had the chance to do this. Hope I helped.

- Thx to Ryan and Jodie for helping out.