I went through this drill last summer. My heater core blew when the temp outside was consistently over 105 degrees. Huge mess. But I got the car home and replaced the heater core and o-rings...but I had a new leak...the friggin plastic firewall coupler was cracked! IPD #113070 - Heater Hose Firewall Junction Coupler. It looked fine, but once everything was connected, I was soon cleaning my carpets...again. I, too bypassed this plastic/o-ring infested connection with new heavy-duty heater hoses and new clamps.

FYI: My heater hoses appear to be aluminum & rubber, not steel, so i have no corrosion issues.

 

Yea, those lines are aluminum and will not corrode. A lot of people take the lines loose at the firewall and pull the box lines and all. I think I also did it the first time or two. I just pull the screw at the bottom of the core case and work it out leaving the lines alone. Messing with those old plastic junctions will cause a leak every time. Heck, I still haven't figured out how to disconnect the lines from it anyway.

Several people have removed the junction and ran hose to connect the lines through the firewall.

 

Yep, exactly. In addition to the core itself being bad, the lines were very very corroded and the plastic clips connecting the lines to the firewall on the inside broke.