I just installed a Z120 into my 996 about 2 weeks ago. Your going to love it. Fairly simple install considering I was replacing a Dual Din Eclipse model which had been professionally installed so all I really had to do was snip the wires to my existing unit and wire it to the Pioneer harness. Easy enough right? Still took me 4 hours to install. Getting everything to fit, hiding wiring and securing everything always takes longer than you think, and I'm pretty particular about my car and how it looks so I took extra attention. Since you have instructions and a wiring harness you should be good to go. All of my wiring into the vehicle (brake/speed sensor/factory speaker locations) came off a factory wire harness, so once you find that, its just a matter of determining which wire is which. For my install, I also have a JL Audio 6 channel amp in the "trunk" which powers all of the Focal speakers in the dash, doors and rear factory locations. I put a custom subwoofer enclosure behind the rear seats with 2 8" subs powered by a dedicated 500w amp also mounted in the 'trunk'. I like all sorts of music, but I prefer my bass tight rather than boomy (as if I had a choice - the 8's fit perfect and anything larger wouldn't have looked right). That being said, in the small space I have, with the power I'm driving, it will boom. It also delivers tight bass with a bass drum that sounds like a bass drum and will play loud and clean until it hurts without distortion.
I fabricated a false floor in the trunk and put the amps underneath so I could keep as much space as possible for storage (hey we need all we can get!) and also have clean, short power connections to the battery and ground. Ran all the RCA and control cables up underneath and into the dash which was a pain, but there is room. The amp in the unit is actually pretty clean, but you've got the 6 channel which will be better. Great choice. Our cars are small inside so they dont need a ton of power to be loud, but I like to be able to hear the music when my windows or sunroof are down, and of course, when its up I want to be able to turn it up on occasion (I try not to - I'm 43 and already loosing some hearing from too many loud systems and concerts for too long....I know I should know better but I love music and its hard not to listen to it loud when it sounds SO good.)
Onto the install - sorry!
The key thing you will need is the mounting cage and bezel. Once you take out the factory deck and move the AC and storage units below (you will need a Porsche factory bezel to move the AC - let me know if you need the part number), you will find the opening doesn't support the traditional install type where you are just going to be able to place the cage and bend the tabs to hold it in. I highly recommend the Connects2 CT23PO02 double DIN install kit from Sonic Electronix.com. Its 129 dollars, but it will come with everything you will need. If its cheaper, or you dont want to wait, it will work well. It doesn't come with any sort of wiring harness, this is just the mounting kit. Once its in, it looks better than factory and will give you a ton of advanced features that you will love (which you already know about since you chose this unit!).
http://www.sonicelectronix.com/item_26489_Connects2+CT23PO02.html
I ended up screwing the cage in to secure it, it will take some trial and error to find the correct alignment and position so that it sits flush when everything is put back together as well as to find the correct position that will allow the bezel to be secured when it clips in. Too far one way or the other and it will not look right.
Note of advice, put a towel over your gear shifter and center console to avoid scratching it or the plastic around it as you install. The picture on their website shows only one bezel, it actually came with 2. Great looking bezel. My previous Eclipse install forced me to actually fabricate a bezel (yeah - good times on that. Cut out black plastic facing material and built it up with body filler compound to the right shape...sand sand sand..paint...sand...) Which turned out very nice but about 8 hours of work just for the bezel. The one that came with the kit looks factory. I've attached a very nice custom Porsche wallpaper/load screen that I think is great - enjoy it.
I ended up putting my XM receiver and GPS receiver antennas on either side of the alarm light location, center and top of the dash against the windshield and ran the cables behind and around the doors, then underneath the dash. I would recommend the XM receiver versus the Sirius. Same content, but with the XM receiver you will get the art for each channel which is nice. I put my XM receiver 'box' underneath the dash on the floorboard in the center, pull gently on the covers and you will find a spot where you can mount it. Its tight so you dont have to screw it in, once you cram it in there, its not going anywhere. Mine is fairly high up the floorboard so you can see it if your looking for it from the passenger side, but I'm OK with that. Overall install of the box and the antennas came out very clean and is a good location for both. I attached the mic to my sun visor, you can easily run the wire down the side pillar and around the door and under the dash in the same way as the antennas for the gps and XM. Mic location for me works great with the visor up, and if I need it to be more clear I can just tilt it down slightly but I don't find that I need to.
I also bypassed of course. Really great unit, your going to love it. I use it with a iPhone 4. Only annoyances are having to goto the nav screen and press OK before it will index the voice response (why cant I just disable this if my music hasn't changed?), inability to turn off the clock display on the unit (I already have a clock on my dash, thanks), and I wish I could just turn off the voice turn by turn directions instead of turning off/down all the guidance. ( I can read the display, I dont need a play by play). Finally, I wish I could easily change the display from day/night for those times where I want to turn on my headlights during the day without dimming my display. Yes I realize that I can turn the brightness up, but I miss the one button convenience I had in my previous unit. If anyone knows how to fix or get around any of these I'm really appreciative of any advice or direction that I might have missed! 
All small things, iPhone bluetooth phone for me works fantastic, iPhone music both connected (use the actual Pioneer cable) or bluetooth works great. VR had a bit of learning curve but I've got it now and having a 6 speed its nice to have the ability to go completely hands free both with the phone and finding music when I'm in town or traffic. The touch screen had also a bit of learning curve, especially when trying to browse the music on my iPhone. I found that instead of trying to 'flick' it, if you take your finger and hold it to the screen while then pushing (keep your finger on the screen) either up or down, it will work much better and avoid selecting the artist your finger first touched rather than moving the menu up and down. It will also allow you to control the browsing speed better.
Some dont like the Navigation. I like the Nav myself (if I could only turn off the voice prompts). It would be nice to customize some of the colors, and in some respects there are things I would improve, but all in all it gets me where I want to go. I use the AVIC feeds to look things up on my iPhone and transfer them to the Nav which works great. Pandora works great, but between the music on my iPhone and XM, I have not used it nearly as much as I thought I would, but I love having it (wife loves it). Sound quality is excellent. Must be the higher voltage pre outs, or just a better processor and cleaner electronics, but it was a remarkably improvement over the Eclipse unit which was what I thought already excellent, and way more flexibility (sometimes too much it seems) in terms of managing the subwoofer and the rest of the soundstage and tone. You have loudness, subwoofer, bass boost and equilizer as well as cutoff frequency control, which is all necessary if you have a strong sub to find the right balance between pop/boom and power versus overwhelming the rest of the sound stage.
I prefer to have too much than too little control and I don't struggle to find the right levels.
All in all very very happy with the unit and the install. Factory/Stock look with high end advanced features. Highly recommended. If you have specific questions, feel free to hit me up. I've installed a number of systems in my time, however I'm not a professional. I've also had my 996 apart more than a couple of times so very familiar with it. Good luck and lets see some pictures when your done!
