I've installed one. Let us know which headunit you go with because some can be hard-wired instead for a more reliable signal/connection. Depending on the headunit, you can go with either the SWI-X, or the SWI-ALP (which I use). Either way, the signal wire for the steering wheel controls is under the steering column in a 40-pin connector. The wire you're looking for is in pin D8, and it is a dark blue color. If you want to use the switched ignition power at the steering colum (on the same wire harness as the steering wheel control signal), that is a yellow wire on PIN A3.
this is a link to the SWI-X which is more universal than the SWI-ALP. (only a handful of units are supported on the SWI-ALP.)