I guess an answer from a police officer is the best advice that I can get! I didn't leave yet. I'm leaving in one months from Montreal and go to the western coast of USA (Seattle, San Francisco, New Orleans, Chicago), and also passing by Vancouver (at least, you can have an idea of which state I'm going to pass by).
So I guess I should get another seat with a seat belt, or put any seat belt back in the vehicle where there are some place to seat (which is a bench seat). Because If I have to plug another seat, I guess I don't know if I'm gonna have enough space to do so. Or maybe a jump seat is the best option ?