Blower Motor replacement created vibration problem

1997 Buick Century Custom 3.8L V6, 106,500miles
The blower motor rusted and was making horrible groaning/squeaking noises. I removed it and purchased a Siemens replacement part. The replacement part did not have the GM standard female connector plug end, only 2 wires, one red, one black. Inside the car the wires were purple and black terminating in the male plug end.

I assumed black would connect to black, and I managed to pull the old female plug connector off of the other blower I removed. I believe I connected the old plug correctly to the new part's wires (soldered, red to "+" and black to "-"), however now the air conditioning vibrates a lot more on highest setting than the old blower motor (lower settings are ok).

I used the rubber gasket from the old motor to mount the new part. And everything is back in place correctly and tightened down, so I do not believe it's a mounting problem. The only thing I can figure is I may have put the plug on backwards, that the red wire on the new blower is not + (though I have no way of knowing because the instructions with the part do not specify which wire is which). Is it possible for the blower motor to "work" and air conditioning to blow normally (except for vibrating more) if the connection is done backwards? Based on what I see in the repair manual electrical diagrams, this looks quite possible.

(EDIT: I checked the old part, looked at the new part and the direction of rotation, and it's rotating the right way. Must be something else.)

Any other ideas how I may have either done this wrong, or is the new part faulty?