If you get really desperate, you could by-pass the heater core altogether.
I've done this on my '97 because the heater core leaks.
I wasn't able to disconnect the heater hoses at the firewall so disconnected them at the front of the engine.
Got 2 lengths of heater hose (one @ 1/2", one @ 5/8") and a brass connector from ENZED
In this pic the original heater hoses are still there, just not connected.
You can see the by-pass hose routed around the air intake tube.
Red arrow is the connector from ENZED.
Yellow arrow is where the by-pass hose re-connects.