Something to attach actors to actors while simulating physics?

I have been developing a game and it is in its “Vehicle” stage, where players can now build vehicles. Everything works except for one thing. I can’t attach actors to other actors while simulating physics, and the Physics Constraint doesn’t work very well because it wiggles a lot. I need the vehicle built to appear as one object to the player. So are there any ways to do this?

Thanks in advance.