The driver's seat will only operate when 1 of 2 conditions are met.

1. Driver's door opened and key in off position.
2. Driver's door shut, and key in on position.

If it works in scenario #1, then the problem is that you are not getting the key in the on position power....

If your ignition switch is worn, you can sometimes get power restored by moving the key slightly to one side or the other while the key is in the #2 (or on) position.

Try holding the seat control on, then wiggling the key.

As a followup question, when the seat fails operate when the door is closed, will your HVAC fan come on? If no, then your problem is the ignition switch, if yes, it's the control box.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles