heater fan problem

I am still working on a mustang telehandler. We found the loose wire that made the forks not work now we are working on the heater fan it will run on low but as soon as you hit the high buton it blows the fues if we put a 20 amp fuse in it works fine do I risk damaging something else if I just leave the 20 amp fuse in. Or is the motor toast.
Thanks for any help