I've seen reference to someone's DIY about modding these.

Take a look at this thread: http://www.peachparts.com/shopforum/showthread.php?t=295241

To maximize re-circ of in-car air and minimize outside it looks like you need to open the in-car flap and close the outside air flap. Going from memory, I think the outboard one is the in car flap (pulls air from the footwell?) and the firewall is the outside flap (pulls air from the cowling?)

Per the diagram, maybe the color of the vacuum lines will help? Green shows in car, red shows outside.