Do you know what level of functionality she has? Because what worked for my son (aspergers, though at the time he was undiagnosed) was to state the rule (*seat belt must be on when we're in the car*) and never start the car until everyone is buckled in. When he was much younger (2?) he managed to figure his way out of anything (car seat, crib, stroller) so we bought an extra safety harness that worked for us.
If the child is at a level that she understands rules, then with reinforcement, that might work.