More on the quick-fix or work-around involving the heater valve.
The heater valve is accessible via the passenger side rear wheel arch (remove wheel).
The Heater valve may also be accessible from above via the engine compartment(remove cover and grope).
Here is a kind guy who posted a video showing replacement of the valve so it gives all the grimy details:
https://www.youtube.com/watch?v=SC198GztmvE
The important part of the video is where he shows how the 2 halves of the plastic valve just fall apart and dump coolant !
Please note the stock plastic heater valve tends to separate where it is glued together, then leaks . That could cause a catastrophic engine failure.
Napa sell a vacuum-operated part that looks similar to the OEM part and is mainly metal with a better crimp:
BK 6601410.
But Four Seasons or Everco sell an all metal vacuum controlled heater control valve that may fit??? It is part # 74671. This valve also fits Audis + some 90's Fords according to Posts in other Forums. The Porsche 944/968 guys use it as an upgrade.
An alternative in all metal intended for 12mm i.d. heater hose is to search "Van Heater Control Valve Kit C3UZ18495A YG133 For FORD E & F SERIES BRONCO DODGE" Just rotate the lever to either block (summer)or allow(winter) coolant to reach the heater core.

To understand why this hack may be a delightfully easy suggestion , read this lengthy, difficult how to:
http://986forum.com/forums/performance-technical-chat/42802-interior-vent-debris-2.html#post323947
There are some useful links there posted by Mr.Stone.

Another detailed link of the full repair process.
https://www.scribd.com/document/212147792/986-Heater-Flap-Repair-Boxster-foam
Another from Forum Member Meir:
https://www.dropbox.com/s/hx9qr0t3jtkv6th/986%20Heater%20Flap%20Repair.pdf?dl=0

There is a simpler way(78F350 in post 9 below) to access the offending flaps in this link and also in this thread:http://986forum.com/forums/performance-technical-chat/42802-interior-vent-debris.html
The simpler system link does require cutting and this is clearly shown in the hi res photos.
You need this tape to do the job well: 3M Flue tape
https://www.amazon.com/gp/product/B00004Z4DS

But whatever you do, replace the risky OEM part to avoid the leakage

Gelbster, anyone who can't follow the simple instructions I posted earlier in this thread for accessing and repairing the HVAC flaps behind the stereo unit probably shouldn't attempt to take their wheels off or look for heater valves. I can see potential for damage. Besides, why disable the unit when it is easier to repair?

It isn't disabling it.It is a seasonal shut-off.Manually operated Was common on older cars .
At least replace the aging heater control valve?
The listed parts will operate with vacuum exactly like the original part and last longer,without the P-car tax:
"Napa sell a vacuum-operated part that looks similar to the OEM part and is mainly metal with a better crimp:
BK 6601410.
But Four Seasons or Everco sell an all metal vacuum controlled heater control valve that may fit??? It is part # 74671. This valve also fits Audis + some 90's Fords according to Posts in other Forums. The Porsche 944/968 guys use it as an upgrade. "
The OEM part # is (but check!) 92857457303. It is $30. But it is all plastic and glued together.

I'll have to have a look one of these days. ;)

Sorry, particlewave, I did miss your point. I guess I have been fortunate. I have had my 986 for five years and up until I did the process I previously mentioned I had quite a bit of foam come out of my A/C vents. As such, it is quite possible that most of the original foam is no longer in place. Having said that, I have not noticed any drop-off in cooling from the A/C even during our scorching South Florida summers.

My foam is long gone. AC still works great but my heater is not as effective as it once was. Luckily, here in FL, heater is effective enough.

I think that I understand your intent. I just don't see the need. I checked through PET and did some searching. I don't think that our cars (986s) have that valve. The circulation through the heater core is constant and not regulated. There is a similar vacuum controlled valve for the transmission cooler on the tiptronics, but nothing for the heater core.
Sure, you could put a manual valve in and operate it seasonally. That brings back memories of working in government buildings that had a date set to change from heat to AC and back regardless of the temperature. I did in fact have my 1978 F350 rigged for manually opening a flap under the hood to circulate through the heater core. My Boxsters all operate correctly via the climate control panel.

Yeah, I definitely don't have that valve and the heater core is always hot.
I think 78F350 is correct. That valve appears to be for the tiptronic.

Thanks for the DIY, by the way. I never planned to do the repair through the heater core access hole because it did not address the other flap. With your method, I had it done in less than 2 hours and I'm now foam free! :D

Thanks guys. My sources show the Tip with 2 of these valves - 1 for the trans and one for the heater. I presumed the manual trans cars still have one for the heater.Why else is it labeled a "Heater Control Valve" ?:
Just search for ECS Meyle 92857457303.Sorry Pelican, your site search did not yield anything except for a 944.The Vertex site confirms it fits my 2001 Bxtr S 6 speed..
Another issue is that a Thread contributor(see 26 below) commented that the lack of foam seemed to have no effect on his a/c in Florida. That seems strange.
So maybe back to the original plan. Find an easily accessible spot to insert a seasonal valve on the input side of the heater core ?

That is weird...maybe Porsche added one to all models in later years?
Or maybe I'm just missing something :)

Did you or anybody else try the cutoff valve for the heater core? If so how did it work? Thanks