Odds were in favor of the heater T's.
Much easier fix than a heater core! Pretty cheap too.
Members have said that rotating the T while pressing in on the lock tabs will hold the tabs in and allow you to slip the T off.
I did not know that when mine snapped at the hose.
I snipped at the the strip of plastic behind the tabs with small wire snips and busted the plastic with a flat screwdriver to release the fitting. Made removal easy-peasy.
If you do that...Just make sure you are busting the right fitting, if you are not replacing both at the same time.
I crawled out to get tools and when I got back under the hood, I found myself with hands on the wrong fitting! Glad I did not bust the wrong one by mistake!!