Who would have thought that reversing this step would be the hardest part of the process? Man, that's hard to hook. :-D

Pro tip: During reassembly, put the plastic backing from the battery down on top of your display, put the trackpad upside-down on top of that, and slowly close the top case down onto it, working the cable through the slot. Then, when it is almost fully closed, use the plastic backing to maneuver the trackpad into position so that the pins line up.

Mine are made of aluminum or stainless or something that magnets don't attract. Flypaper, maybe? :-D

I found it, and it wasn't in a place you might expect. There's a bit of metal trim on the back of the trackpad itself, and the washer was underneath that. I actually had to dump all the washers out onto the bottom case, and suddenly it peeked out from under the metal rim. Worse, these things are aluminum (or maybe metallized mylar), so you can't pick them up with magnets....

It might be a good idea to put some masking tape on the trackpad to hold it in place so that it doesn't drop out when you open the lid. Then very carefully press it down from the underside with a spudger after you have opened the lid fully and have your hand under it to catch the trackpad so that it doesn't suddenly drop.

This needs to be moved WAY up higher. You are 100% guaranteed to lose one by the time you start the previous step, because the trackpad will fall the microsecond you start to open the case. Really, you shouldn't remove the last screw until you're holding the trackpad from underneath.

Also, the battery replacement kit should really come with at least one extra washer, because I can pretty much guarantee nobody will ever do this without losing one. It is darn near impossible to deal with something that lightweight. Even putting the trackpad down on the counter is enough to lose one, much less having it drop out of the bottom while you're cracking open the laptop.