I used the simplest solution to the 'too short' heater core cord.
In my 2004, the cord runs up inside a split wiring 'shield' and exits the shield quite high. I just removed the cord from the top 18 inches of the split wiring shield, cut a small notch into the split of the shield where the cord now exits, and with straps attached the plug onto the passenger side tow hook, facing forward. All I do now is push the house extension cord onto the now solidly attached heater core plug.
Worked for me.
Tor
2004 GMC 2500HD SLE Duramax, Allison sb cc
2016 Acura TLX SH-AWD Elite