I had a similar problem on my E320. I replaced the faulty regulator with a new one. Since it was the updated regulator I also had to replace the blower. I kept the old blower, and I just tried your test. When turning it by hand it spins quickly then stops quickly after a second or two. I'm not sure if that helps you or not.

If you're regulator really is OK, I'll sell you my old blower. It would save me the trouble of posting it on ebay. Send me a private message if you're interested.

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1998 E320 120K mi
1991 Miata 80K mi
2001 Ranger 109K mi