What all have you tried to remedy this? My guess is it would be the vacuum pod in the center of the dash. The rubber diaphram in those pods can crack and let vacuum leak through causing the vent blend door not to close and allowing air to bypass to places you do not want it.

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2005 Accord Hybrid (Wifes)
1995 Subaru Impreza L AWD (Snow car)
1984 GMC Sierra 1500 (Mine)
1983 300CD Best $ I ever spent. (Mine)
1984 190D (sold and glad I did)
1983 300D (sold and wished I hadn't)