That youtube search also gets you to a video of a shortcut to replace the heater core. I’ve never had to replace one, but it looks like a fine way to do it when the time comes. I’ve always had good luck with flushing a plugged core with a 50-50 solution of CLR. I don’t know what your daughter’s issue is, but the blend door actuator is only one small part of the system. On those I’ve had that broke, they could be heard clicking and clicking and clicking behind the dash. Follow the video for blend door replacement and quit at the actuator stage if you need one, but first check the fuse that runs the actuator.