Simon,
I would suspect the blower switch, as the current going through it is really too high for its rating. As the aircon condenser fan and mag clutch are also wired in series with the blower (to ensure that the blower is always blowing whenever the aircon is on), you can add quite a bit to its normal drain.
When the aircon is on, my blower fans take:
Low - 10A
Medium - 10.6A
High - 12.5A
Rather too much for a 5A switch!
May I suggest that you first bypass the blower switch and see if the fans run when hot wired from the 12V supply. Incidentally there is often a line fuse in the supply line; GTD don't include the blowers/aircon in their wiring diagrams, so there isn't a fuse for them in the fusebox. If yours isn't a GTD, ignore all the previous sentence!
When you have got your system working again, I strongly recommend you put 4 relays in the system to take the load off the switches. Premier Wiring Systems (01483 236976) do a series of fused relays that clip together into a bank for easy mounting.
You can put whatever fuse you want into each relay to match the circuit it's feeding and your switches will last forever. Cool. /ubbthreads/images/graemlins/laugh.gif
