The last step was to stuff everything inside and close up the kickpanel. I knew from my previous limited experience working on things like this that they don't always go back together the way they came apart. This turned out to be no exception to that.

I tried a couple different ways to zip-tie my little box into place inside the dash. I had already spliced in some extra wire for the switch end and the buck converter end to give myself some maneuvering slack.

Unfortunately, every place that I tried to put that box seemed to get in the way of closing up the kickpanel. In the end, I let the little white box sort of float and be out of the way. It's plastic, and I hoped it didn't lead to an annoying rattle. Later testing showed that so far, so good. I also hope that it doesn't someday stress the soldered or crimped connections in the wires to break the circuit.

Here's a look at the final result. 

My 3D printed switch blank doesn't look completely like the other switches, but it's also not especially visible in normal use.

Victory!