What do I have to do to drive another car that isn't mine and doesn't have it?

like, in the level 2 mission 5 where you start in the mission in the cletus car with cletus driving, how can i do it?

You can't use the car at your own leisure without unlocking it first. You can use the car in the mission if it forces you to, but otherwise you need to unlock the car.

If you're looking for a mod to do this, in the hack "Debug Test" under the "Developer" tab in the Mod Launcher, you can enable this setting:

thx.
but in a mission, how can i force the player to drive certain vehicle?

Oh you're referring to a mod script. Someone else would have to help you for that, but I can give you the basics I know.

You can look at scripts/missions/level02/m5i.mfk and scripts/missions/level02/m5l.mfk for information.

Here's a screenshot of m5i.mfk, the highlighted areas refer to setting a forced car.

In your loader script (m5l.mfk) in this example, you need to load the car:

LoadDisposableCar("art\cars\cletu_v.p3d","cletu_v","OTHER");

yeah i have a problem with the _sd, i couldnt find it in the files of the games, do you know where it is?