So. I did a lot of poking around because I always meant to and if this isn't the cable I would be surprised. I will have to do a little more poking around to be sure, but the haptics on this are driven by a chip on the trackpad itself. I presume it also talks to the touch controller (which is also on the underside of trackpad) but since I don't have a schematic for the touchpad itself, or the little interconnect board the keyboard connects to it's a lot of extrapolating based on datasheets and what I can get from a multimeter.
I would absolutely check the cable and make sure there's no liquid that got into any of the parts of the cable, or that the cable isn't bent or torn anywhere.
Apple also calibrates those when they do the repair themselves, but I have never seen that make a specific difference to function so I am unlikely to think its needed.