Several months ago I decided to replace my A/C evaporator. I had a slow leak and It's 31 years old. So, I pulled the dash and replace the evaporator and the heater core, also 31 years old, and put it all back together. I had a new blower motor that I bought years ago so I replaced that while I was there. Everything was fine until a few weeks ago, with the warmer weather moving in, I took the truck to the local garage to get it evacuated and refilled with Freon. They serviced it but notice very little flow from the blower.
I have been trying to figure out the reason since then. Finally, I pulled the blower and it ran fine with 12V so I decided to put the old blower back in and wal-la . The new blower must be wired backwards from the factory.