Hi N.,
My son, now 3 1/2, started unbuckling his car seat at 2 as well. I tried the rewards system, talking to him, having other people talk to him nothing seemed to work. I can remember one time when I sitting at a red light and just as the light turned green the little bugger unbuckled and jumped into the passenger seat and said, "Hi Mom" with a big smile!
The only thing that worked for me was to stop the car every time he unbuckled and I would not move until he put himself back into his seat and buckled back up. If he could unbuckle then he could buckle himself back up. Just to make sure, I showed him how to buckle the first couple of times. The first couple of times after teaching him how to buckle were tough. I would sit on the side of the road for sometimes 1/2 hour. As soon as he realized that I wasn't going to be buckling him in he stopped. I would say within a week he had stopped unbuckling. He only unbuckles when the car stops at our destination now.
Good luck. I know how frustrating it can be!