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