And to follow-up, it works perfectly. Sebastian, you made my wife very happy.
The supplied 3.5mm male-to-male cable protruded just a bit too far in my configuration to snake out the side because of where I stuck the Leptronix board to the AM/FM module; as a warning to others, try test-fitting everything before you actually stick the board to the AM/FM module to make sure you've mounted it in a good place for all wires to clear properly. For my own purposes, I just cut a hole in the rear heat sink right behind the Leptronix board, so both new wires can come straight out; that also makes it very easy to replace the audio cable without disassembling the radio.
In auto mode, it switches over to the aux input almost instantly when you start playing something. There's a 15-30 second delay switching back, which is reasonable (you don't want it to just randomly switch back to the radio, because you're between songs). When physically switching from radio to aux and back again, it's almost instantaneous.
Audio fidelity is just as good as any CD I've played in the head unit.
There's almost nothing else to review here; this was a blissfully simple installation, and it worked exactly as promised. Sebastian has created a first-rate product.
Now I just have to find a place my wife will approve of to install the switch and female aux-in jack I picked up.
