I ended up actually getting one from Crutchfield - I could have wired the same thing myself, but the form factor and ease of connection for that device made it worth grabbing the pre-made version.

The result worked, more or less... No noise or hum, but I don't think it attenuated enough. Once I start to push the volume up past the first 8-10 notches from 0, I start to hear some distortion in the signal. Maybe I'll try messing with different resistor values or something.