I put in a blower motor (ACM) last weekend and if one does not check before sewing it back together it is easy enough to put the motor in backward. The vibration could also be due to the top cover of the blower motor not being placed snugly with ALL clips re-attached....Trust me...I spent a 2.5 hours last weekend (in my garage thermometer read 22F to boot!)getting that right...Finally after two weeks I got to drive it for the past couple of days with heat!

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Matt
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92 400E 128K
99 GMC YUKON 88K