I have a 264 and wondered about this too, so I took the grill off and it is a heating vent but the tube is just sitting there, I made an aluminium box and connecter the tube properly and it warms fine now.
"Well that didn't go as expected"
2015 Chev 2500HD Highcountry Duramax
Cedar Creek Silverback 33IK
Donald&Casey cairn terrier
Rest in Peace Mary my darling wife.
Scottish by birth Canadian by time.