Have you checked the ignition switch?
I had a similar problem on my 300E. The blower would blow in preignition, but cut out when I had started the engine. Turned out the blower motor and controller were fine the ignition switch was bad and wasn't feeding any power to the controller.
Just thinking this might also help. The ignition switch is a faily cheap and easy to replace part.
Good luck.
F.J

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F.J.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)