Legal Question in Civil Litigation in California

You need to read the terms of the contract through which you got the car. The car repossessor certainly can enter private property, which often is enclosed by a gate which serves the same purpose as a locked garage. Without looking to see if there is any case law, I would guess that there is a good chance it is legal.

As a practical matter, what is it that you wish to do? Did they have the right to repossess the car? If so, they will not pay you any money for a trespass and a jury would not likely award you much.