Replace your monovalve insert, it's likely wrinkled or torn, and isn't feeding much water through the heater core. On Defrost, you get high fan and full heat (Monovalve constantly energized), so the blower will run at 3/4 speed or something.

You will have very low speed fan and cold air (higher fan speed but still cold air on high fan) until the heater core warms up.

Verify that the aux pump works, too, and that you are full on coolant. Also, if the thermostat is bad, it will take forever to heat up (you should run just above 80 C rather quickly).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!