I think I will use an RCA variable level attenuator, aka: volume knob. Now I see why the Belkin dock connector + 1/8" jack + cigarette adapter (which I currently use) has a volume knob; because the dock connector outputs audio at full blast.

It's amazing how much this install is costing in individual $5 and $20 parts. Not as much as that Alpine, which looks really darn cool for iPod integration.