[Jochemd]

You should also lock it for the streaming in vehicle.

pawn Код:

public OnVehicleStreamIn(vehicleid, forplayerid)
{
    if(VehicleInfo[vehicleid][Locked] == 1) SetVehicleParamsForPlayer(vehicleid,forplayerid,0,1);
    else SetVehicleParamsForPlayer(vehicleid,forplayerid,0,0);
    return 1;
}

Edit it for your own script