You might have a monovalve malfunction. The Monovalve is normally open. When powered, it shuts off the flow of coolant to ensure cold air. If it fails, or is partially open, then it will let hot coolant flow resulting in warm air out of the vents.

When the engine is cold, i.e does the a/c blow cold? Your window might be very small for this test.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold