Mine is also under the oven...look for a cabinette door with a wood frame around a black metal mesh in the middle. On my 8280 one of these in the entertainment center provides air circulation to the "basement" area, the other, under the oven, is the furnace. Both are screwed shut.
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2008 (or is it 2009?) Rockwood 8280SS - 2007 F-150
Lot's of mostly Kenwood radios