I too have been searching for the answer to this question. I've recently purchased a 2004 Turbo and would like to replace the PCM with an aftermarket satnav and stereo unit that has bluetooth functionality! Anyone ever replaced theirs before?
Do a google search and you will get a lot of hits covering this. You will also find videos on youtube showing how to do it. It is important to verify that the 3rd party PCM supports the bose sub woofer. Otherwise it does not seem to be complicated.