Compared to the seat bottom, adding the heat pad to the back insert was a piece of cake. I found the holes used by the hog rings were too small to fit the pull-ties through, so it was necessary to drill 1/8" holes in the retaining plastic. The easiest — in fact, the only way to accomplish it cleanly — is to do it from the underside (see photo). The plastic isn't attached, so it can be lifted away from the foam to get a clean shot at the target with the drill, three holes per slit, nine in all.
Whoever installed the original hog rings must have been a neophyte. A few rings missed the plastic entirely. The pull-ties attached the leather to the cushion much more securely. In case you haven't noticed, I'm big on pull-ties for this application.
All that remains is to hook the leather at the bottom and the seat insert will be finished. You really have to reef on the leather to hook it together, due to the thickness of the heat pad. I couldn't do it alone, so I enlisted a pair of strong hands to assist.
Next, I'll join the seat back and bottom by bolting their respective frames together. After taking care of a few details, such as pull-tying the wiring in place under the seat, it will be time to wrestle the seats, both heated, back into the car.