Ford Ka (2001) heater blower, fan fuse blowing?

I replaced the fuse following fan motor stopping.
Started the car to check if it was now working, and smoke came out of the 2 central fan nozzles, although the fan worked!
Started the car again, and the fuse blew immediately I switched the ignition on.
Sounds like a rogue earth somewhere!
Any hints would be appreciated.

jonno
August 2010