One cable, two rods, and a bellcrank, right?
Worn clevis and/or clevis pins.
Worn bellcrank and loose pivot?
It has been a long, long time since I've seen an actual stretched e-brake cable that was not exhibiting other symptoms first, like fraying, rust, sticking.
I'd be looking at the other bits before the cable.
If external mechanicals are okay, does the ends of both radius rods move as soon as you have someone ratchet up on the lever?
If so, I'd look at the e-brake levers and locating pins.