Did some experimentation this weekend. I think you can
avoid cutting a second hole in the face of your rear seat kickpanel by
not installing the cold air return 
inlet.

I used some tape to close off my inlet and noticed
no difference in the output of the furnace, nor any difference in the
sound the furnace was making. 

My theory is that cold air backflows through the 
rear heater and squirrelcage fan; plenty of air
is available to be heated by the furnace without supplying a new inlet
vent.

Later,

Tom F.