def link_model_io(existing):
    """Link I/O drivers back to the models they communicate with.

    Args:
        existing (dict): Dictionary of existing components.

    Returns:
        dict: Dictionary with I/O drivers added to models.

    """
    for m in existing['model'].keys():
        existing['model'][m]['input_drivers'] = []
        existing['model'][m]['output_drivers'] = []
    for io in existing['input'].values():
        for m in io['model_driver']:
            existing['model'][m]['input_drivers'].append(io)
    for io in existing['output'].values():
        for m in io['model_driver']:
            existing['model'][m]['output_drivers'].append(io)
    return existing
