After re-reading previous, on the standard hvac setups, the heat/cold is controlled by an actuator on one side or the other (forget which) and has a plastic rod that extends over to the other side to control that side. I've seen a report where the rod broke. (I think it controlled the passenger sidevand the rod simultaneously controlled the drivers side. Easiest way to check is to watch the heat/cool door on the drivers side and switch between heat & a/c. If the passenger door moves but the drivers doesn't, you found your problem. If both move, the core is partly clogged.
On the duel systems, each side is controlled separately with separate actuators. There is a history of these actuators failing in the Ford line.
Its possible that the core is clogged again. Rather uncommon however. Most common cause is by using different coolants. Mix the wrong types are they congel forming a thick goo, and block part of the heater core. Never read where a flush cleared it. Think it would be better to disconnect the heater hoses and do a power flush just on the heater core. I did this many years ago by connecting the harden hose to one side heater hose and opening the tap. (Back then, Prestone sold a kit, a tee, to tap into your heater hose with a garden hose threaded connection on the top of the tee. Designed to do a flush at each coolant change.) Anyway, it worked. (But I'm not advising. Heater cores are made cheaper today.)
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