Actually got some work time this weekend, ran the pump a bit, and it didn't sound like it was pressurizing. So I went for it, held it down for several seconds. Sure enough it started dripping from the headliner trim. Didn't take long to find a break inside the boot between the body and rear gate.
2021-02-15_08-08-43 by Numbchux, on Flickr
Looked like it was pinched, but I'm not sure how. I pulled a coupler from a parts car, spliced it together, and I have a rear washer again!