I've tried to save a custom car in Trevor's home garage, and it was saved: I left and came back as another character and it was there.
But when I took it for a ride, and then died, I couldn't find it anywhere. I know there were some bugs with this, but I saved the game, and the car did stay in the garage- but when I left it and died, it disappeared (it hadn't been destroyed).
What do I need to do to get my car back?
I'm playing on the PS4.
Saved cars do not remain saved permanently. The moment you take your car out of your garage, it is no longer saved until you put it back in.
The only exception to the rule, is the car that each character owns. Those cars will always be waiting in or next to the character's garage, regardless of whether you leave it somewhere or have it destroyed.
Keep in mind, that in Michael's case, his car will be unavailable for quite a long while when
his wife and kids leave him.
His car will be back and cutomized at a later point in the game, which is why it's not recommended to customize Michael's car before his car is back, as you'll lose all customization done before this point.
It's possible your car is not lost forever, though. As long as you didn't destroy it, you might find it at the impound. You should know where the impound is, if you finished Franklin's side job.
