Note, I have the HVAC box top OFF so your setup should look a little more complete then this. chances are it's your footwell pods or the Central Control Valve. Buy a mighty vac, It will soon become your best friend diagnosing the system. if you have to do the central Pod, you might as well replace all of them while your there, it's about 100.00 or so in parts, but it's also peace of mind

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed