On the 986 open your frunk (front trunk) and stand at the front of the vehicle. Its up in the left hand
far corner right below the winshield (passenger side), right against the firewall. Black plastic cover.
Thats the cover of the HVAC. They make one with charcoal in it that absorbs odor.
Here's a link on how to do it too.
How to Replace a Boxster Cabin Air Filter | eHow.com