You mean the cover on top of the core that is tucked way up under the dash. Well it is basic job is to keep the air flowing through the core and help keep crud out of the core. Without it you will have a large air leak and very reduced air output from the heater.
I do not know if you could get it up in that spot without at least pulling the dash out partway to be able to get the front duct out of the way far enough to slip it into place.
Tuesday, January 17th, 2017 AT 2:48 AM