BRT

I recently purchased a 1965 Dynamic 88 convertable. The fuse for the heat/ac blower motor was blown. I replaced the 20A fuse, the fan ran for a few minutes, and melted the fuse. I replaced the blower motor and repaired a lose connection at the resister, the resister looks ok. I replaced the fuse and the same thing happened. I did note that when the fan was running it did work on the different speeds. Any suggestions on what i should check next?