aside from spraying a disinfectant into the duct work which is the back end of the real problem, you need to be concerned about mold/mildew growing in the evaporator (especially if drain has been plugged up).
There are 2 solutions to the root of the problem. One is a foam spray you use by forcing it up to the condenser thru the drain tube and let it work it's way in all the crevices and kill the mold/bacteria/etc. Not sure if they're Porsche approved but here are some examples of product you can usually pick up in a local parts store or at a dealer. Wurth I think makes a few variants too:
The other solution is simple....... with the engine fully warmed up, run the engine for 30-45 min w/ the heat set on max and make sure the A/C is on (snowflake lit up). Occasionally, rotate thru the various vent settings (to kill any mold/mildew around the blend door and or various ducts). It'll get more than 'toasty' so do this w/ the top down while driving some day. You may have to repeat this once or twice before the odor is gone completely and maybe once a year afterwards....
Good luck
