Ok, well that was kinda more work than I expected. My friend Paul has put this exact same head unit in his '01 996 Turbo and he came over to help me out. His help was much appreciated and this would've taken far, far longer had I done it solo.
First off, set the volume on the stock radio and make sure all the settings are where you want them. Then pull out the radio, it uses the same radio keys that the older VW/Audi's used. Unplug all the harnesses.
Time to take everything apart. Pull out the horseshoe and then remove the 4 torx screws holding down the surround. Remove the 4 phillips screws holding the hvac controller in place, then unplug the two harnesses. Pull out the top hvac vent bit.
Pull off the two covers on the bottom part, the leather wrapped on and the carpeted one. Now pull out the bins or cd holder, whatever you have down there. Pull off the batwing. Now you're here.
Take the two hvac harness plugs and route them to the bottom of the stack. Removing the hvac vent underneath the drivers side makes this easier, make sure you take out the 1 phillips screw holding the vent into place that's above drivers right knee.
OK, now take the Metra piece and start modifying it. You need to cut out the center bar, down the sides plus the tops off the corner screw holes.
OK, lets mount the stereo in it. Here's where things kinda stink if you have a 2003 or later 996. Metra used to sell a kit specifically for the later cars but discontinued it, they only sell the older kit. Problem is they didn't engineer a way to make the kit work with the later cars, nor did they change their instructions to tell you that you need to engineer something.
Basically you need to make a wedge to go in between the surround and the plastic retention brackets they give you. Paul used a piece of plastic cutting board, then covered it in duct tape. It's totally secure now.
OK, time for some wiring. Since I have a '03 I have the fiber optic network, so $129 later I have an adapter box from China. Connect it to the factory fiber optic harness (orange), then wire in the switched power, ground and constant power. You need to rewire the Metra wiring harness adapter too. I de pinned it to remove the things I didn't need. Basically all you need is ground, switched power and constant power. So you can piggyback it with the MOST adapter.
Grab switched power from the unused car phone plug, the wire you want is green with a black stripe. Everything else you can get off the stock radio harness plug.
Ok, lets connect the hvac headunit in its new home. Connect the two harnesses and reuse 2 of the stock phillips screws that held the hvac headunit in above to mount it.
Now install the Metra surround. I grabbed one of the rubber mats from my stock cubbies to put in. On the stock ones Porsche used "dowels" in the rubber mat to hold it down. I thought about drilling/cutting the holes in the Metra piece but decided to just cut them off the stock rubber mat and use double sided tape to hold it on.
Here's where I installed the Rennline phone mount kit. I also put the hands free phone mic in and ran the USB for the Apple Car play. Tested the radio before I buttoned it all up and it works. Time to put everything back together again. You need to slide the upper hvac vent into the radio surround FIRST, then slide the entire assembly into the dash. Use the 4 stock T20 screws to hold it into place. Now reconnect all the harness plugs in the horseshoe and snap it into place.
Ok, now reconnect the hvac vent underneath the steering wheel and put the phillips screw back into place. Now snap the two covers into place on both sides, the leather wrapped one and the carpeted one, snap the "batwing" back into place.
Here it is all completed, I think the Metra surround looks pretty darn good. You can see the mic on the upper left corner of the cubby. The car is so small reception should be fine there.
I'm not happy with the way the Rennline phone bracket pushes the trim piece out. I can't see it if my phone is there, but otherwise it's not flush and drive me nuts. I may modify the stock trim piece a bit to get a flush fit. I was surprised the head unit didn't import my contact list but its possible it just needed more time to work. Apple Car play is working fine.
The sound quality seems better but that may just be in my head, since I retained all the stock Bose amps/speakers from before....