Ok so I obviously fall into the category of trying to fix it. That is enjoyable to me.
I did find that heat shrink tubing is a bit too rigid, even compared to the old stuff that had hardened. Then I went into my stash of wiring bits and pieces, and found part of a harness from some unknown vehicle and year. Turns out to be the exact same tubing, in terms of color, diameter, etc.
The first picture shows the donor harness, with wiring removed from the tubing. The second picture shows the tubing being bent, and a piece of heat shrink tubing (green tape on end) that does not like to be bent so much.
Since one wire from each door was cracked at the flex point, I decided to cut there. Picture three shows the type of wire used. The broken wires were of the 0.013" diameter strand variety. Note that this is an interior décor with convenience control option, no door speakers.
I did an inline splice on each wire on both sides of the large rubber plugs. The donor wire used was all the finer strand (0.010" dia) so as to be more flex friendly.
As Bob noted, the large rubber plugs were attached to the old tubing (glued perhaps), and I glued the replacement tubing in place.