Well, okay. Looks like this will turn into my thread for collecting all the bits of info and questions for this project, rather than starting a new thread for each question.

I got a reply from PAC Audio regarding the SWI-X interface:

Quote:
Unfortunately the Swi-x can only read a single press of a button, not the
release of the button. When a button is pressed on the steering wheel it
will send a resistance signal to our unit, and our unit will record that
signal and match it with the signal of the remote. Most cars don't have a
different resistance of the release of the button so there is not a way to
differentiate the push and release of a button. So unfortunately its one
function per button.

I don't really understand part of that answer -- if you can detect the pressing of a button by a change in the resistance signal, why can't you detect the release of the button by the opposite delta in the resistance? What am I missing, here?