I think it's worth adding that there is an additional way to mod 05+ Impreza radios to add an aux in. The CD audio lines are also available for use inside the head unit via a ribbon cable that connects the CD changer/module to the radio/amp. The technique has been outlined on two threads on legacygt (slightly different approaches) and attached are images of the insides of an '05 WRX radio that show that the internals are identical (or at least similar enough to use the same principles). It has been tested and both methods work on Legacy, Outback, & WRX/Impreza radios.
There is also a technique for using the ribbon cable directly using additional parts.
More poineering members have even cut the traces for the AM radio and soldered aux audio lines onto that, preventing the need for a silent CD, but that's a level of modification most owners would be uncomfortable with.