Why my car with trailer cannot move? (using hinge joint)

The car I use is from unity standard asset. It can move normally. However, after i added a trailer for it and $$anonymous$$nge joint/character joint, the wheels are spining but the car cannot move.

Both of the car and the trailer have 4 wheels collider and box collider.

If I set a very small mass of the trailer (not a good solution), the car moves. It seems too heavy for the car.

I have seen some one have the same problem. He use a script to solve, but I dont know the language and the script content.

The link: https://www.youtube.com/watch?v=cRBUDsZ5uJk&t=627s

The car:

The trailer: