"defrost tubing" to the vents up towards the inside windshield?
There were two styles of GM accessory heaters:
1. Recirculating - works fine recirculating inside air for heat and/or defrost. Easier to install and restore. Less expensive - $50 for one needing restoration, but with a good motor (just strip and paint?). Less glitzy, but does the job.
2. Deluxe Air Flow (Fresh Air) - works fine using inside or outside are for heat and/or defrost. Harder to install and install. Much more expensive: $75-$100 for one needing restoration (and missing hard to find parts) - $250++ for a properly restored one. There were two similar styles: 1947-1952 with no interior input-air door, and 1953-55 with interior/exterior input-air door)
People will post links to photos of both styles.
http://www.1954advance-design.com/Documents/Accessories/Booklet/Page1.jpg
