If you can't find the Alpine modulator (discontinued), Scosche has one available as well. The Scosche FMMOD02 is a very straight forward install and is only $50 from Crutchfield. You still need the 12040VW21 and 12040VW12 adapters (also available thru Crutchfield) to hook it up to the Infinity nav unit, but it's a simple red wire/black wire power run. Shouldn't take any more than 30 minutes, tops. Most of that time is running the power wires.

This will work with the Infinity nav unit, but I'm not sure about the non-nav unit as I don't know what the back of that unit looks like.

This is a good alternative to the $200+ unit for sale on the site.