Here's what Old Air Products says about their hidden AC duct system..............
"Custom louvers, blower switch and thermostat mount behind original speaker grill. Cables for Defrost & Heater functions can be mounted in unused choke & throttle holes or under dash. NOTE: installation requires 1947-53 speaker grill w/ hoizontal slots,"
Link to catalog
http://www.oldairproducts.com/catalo...ge-p-4485.html