I have an 06 wrx limited with the 6 disk, but not the aux input. Could i swap this with an 07 one with the aux input and be plug and play, or is there a difference in harness?
I do not know the answer to your question...however, I do have an OEM head unit out of my '07 STi with the AUX port. I actually never even tried to use the AUX port as I had a 7" in-dash unit to install right when I got the car. Make an offer if you're interested
After a brief google search, it appears as though the AUX jack will not work with the '06 head unit. The '07 head unit has a CD/AUX button that the '06 unit lacks.
I re-read your OP lol...I thought you just wanted to add the AUX port to your '06 unit, which you cannot. To answer your actual question, the '07 head unit and AUX port is a direct plug-n-play swap into your '06.