I didn't say it wasn't possible, I stated why its not a good idea. If you install an aftermarket radio you must wire around the AFBS, since it won't work with the new head unit. When you do that, you will be hooking up the new HU (designed for 4ohm load) to the stock rear speakers (2 ohm load) which will: 1) sound distorted and unclear, and 2) eventually blow your speakers.
If you insist on doing it here is the link for the work around:
bypassing AFBS system (5th gen)
Note that in the writeup it even says to run aftermarket speakers