I tried updating my photo and trying to expand my posting acumen so I can make a decent "how to post" on the heater core and dash. There are two of these "hooks" (white arrow)on this connector. One on each side of the connector. I think you need two hands to open each hook, and then another set of hands to pull the connector apart.
(Thanks to the people who provided the tutorials on how to post. Couldn't have done it without you.)