Black foam from heater - some notes from my experience
 

Both my Boxsters have this problem, the one I am selling is the worst. I decided to fix them both starting with my 2000S, that I am keeping, to gain experience before doing the one that is for sale.
The video from Help Me DIY is my inspiration:
https://www.youtube.com/watch?v=42IZ0lb33C8
Some things that I did differently.
1) Instead of cutting the sides of the snorkel-like duct and folding it down I cut the bottom as well and removed the cutout completely to simplify the operation and have more room.
2) I worked with the first door partially or fully open rather than fully closed as shown in the video. This gives more room to get a hand in to remove the old foam and the access to the door is just as good.
3) Instead of using tape or foam I used a piece of thin aluminium that I JB welded to the door. The advantages of are that the aluminum is easier to control than tape when maneuvering it into place and before the JB weld hardens the patch can be slid around to place it accurately. With the door fully open it is easy to feel how the patch is placed at the bottom and on the sides.

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I stuck a couple of pieces of masking tape with a fold in the middle onto the patch to act as a handle during installation.

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The second door will not be so easy. The foam on it is starting to degrade but is almost intact and very resistant to being removed. I am considering the alternative of going in through the heater core which gives better access to this door. I can wait to do this later, so will button up the parts that I removed and come back to it some other time.

I believe there are 3D printable plugs out there, worst case I could design some. I assumed my car would have the same problem but when I opened it up the foam was pristine.

I saw the plugs but I think installing one aluminum plate is easier than installing the the plugs, especially the lower ones that you can't see.

I found this video, it is very detailed, both going in from the front and from behind the radio.
https://www.youtube.com/watch?v=ncCc-ZbdaFI
I don't understand why he didn't cut the duct behind the radio all the way up. Access is improved by cutting all the way up.

Fair enough, I didn't actually get that far in the process.

I did this "repair" a year ago with silver ducting tape (not actual duct tape but the more rigid metal tape). It has failed. It rattles :|

I think it may be where I taped the vent back up mostly, but I'll be adhering in the plugs, probably going to print them out of PA6-GF.

I'd be interested to know what you find when you go back in there. I too used the aluminum tape to close up the duct. In Help Me DIY he used Hi-heat aluminum tape, rather than brand X like I did, maybe the difference is important.

I would instead go softer and use TPU, easier to get in, presumably, and they shouldn't rattle. If they have the right profile they shouldn't even need adhesive.

Going in from the frunck I got the front blend door out... it was still almost fully clothed in foam, that was falling off. The foam was on both sides, as others have indicated and I think even folded over the edges. So, doing the repair with tape or an aluminum patch as I am doing means that there will be leakage around the door where the foam should seal it. I am debating whether to add foam around the edges to get a factory like seal. Probably not...

I also accessed the blend doors from the frunk when I did this repair as that seemed to be far less invasive that cutting the ductwork behind the head unit. It is possible to recover both blend doors this way although I'll readily admit that getting to the inner door was a PITA. I covered the doors with craft store self-adhesive EVA foam sheets. This has worked well for me.

You can see how I did it here - https://rx19jones.wixsite.com/projectboxster/projectboxster7

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