spoilers...don't look here if you haven't beaten the game.

[Cavalry]

I just finished the final mission, but still haven't been text messaged by Stevie about delivering cars to the garage, when is he supposed to start texting you?
I finished delivering all of the cars for Brucie, but where is Stevie?

 

[knut]

You might of got the text earlier in the game but forgot about it. I got the text well early into the same, as I only remember the guy existed when I was watching my friend play the other day and he got the text around the 34% done mark.

Obviously it may vary, though. I'm not quite sure.

 

[Raziaar]

I got the text but I didn't know where to start. Was I supposed to just find the type of car?

 

[nokori3byo]

I got the text but I didn't know where to start. Was I supposed to just find the type of car?

Nope. It's the specific car shown in the photo attached to the message. That photo will also give you clues about where exactly the vehicle is found. A couple of them are quite tough to find, but you can always google up a map, if you're having trouble.