[ReneG]

You have to use OnPlayerExitVehicle, and OnPlayerState, it only gets called when a player is actually out of the car, at which point he has no vehicle ID. That's what I think is causing your code not to work.

Under OnPlayerEnterVehicle, you get the seat id and store it into a global variable. Then on OnPlayerExitVehicle you put them back in the same vehicle with that global var that holds the seat id.

pawn Code:

new
    // at the top of your script
     gSeatID[MAX_PLAYERS]; // a global var for the seat ID

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    // if either they entered as a passenger or driver
    // adjust their seat it
    if(newstate == PLAYER_STATE_DRIVER || newstate == PLAYER_STATE_PASSENGER)
    {
        gSeatID[playerid] = GetPlayerVehicleSeat(playerid);
    }
    return 1;
}
public OnPlayerExitVehicle(playerid, vehicleid)
{
    if(Player[playerid][Seatbelt] = 1)
    {
        // so if the player exits the vehicle
        // and they're still buckled up
        // you put them back in the vehicle
        // in their global seat ID
        PutPlayerInVehicle(playerid, vehicleid, gSeatID[playerid]);
        /*
         *  You would put the code that notifies the player
         *  that they are not unbuckled yet.
         *
         */
        return 1;
    }
    return 1;
}