I have purchased 3 used raptors so far, and all 3 had a broken base of the reverse knob that 6pack mentioned. The repair plate works great thoughHave you checked the reverse knob?
It's common for it's base to crack or break.
May or may not be your problem, but worth checking, and a $10 repair plate if it is.
It would be unlikely for a reverse cable to stretch, given it's low stress and limited use.
Have you tried adjusting the cable?
The reverse shaft coming out of the case, where the cable attaches, also has a spring on it. Check that the spring is installed correctly and has not broken.
You should be able to adjust the slack out there then, there is a jam nut against a hex sleeve that will tighten or loosen the cable's slack.I know theirs slack at the bottom where it meets the case and I figured because of it, it wouldn’t engage but never tried fixing it. I’ll take pictures when I pull it out the trailer