No, it's probably disconnected because it's bad. Only reconnect if you can verify it's not seized and doesn't draw too much juice, else you will be buying a replacement pushbutton unit, too.

Yes, you do need it, otherwise coolant circulation through the heater core is pretty pitiful at low speed, and the heat will surge hot/cold.

You should also check the monovalve -- unplug it, you should get full heater core flow. If not, probably stuck.

It's also possible there is something wrong with the pushbutton unit, but if it hold temp properly on AC, it's likely OK and the problem is the monovalve or the aux pump.

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!