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[130CC]

The standard for wiring in a car is that a switch such as the demister one is only used for switching low current of a relay and the like, the high current the actual demister circuit is switched by a relay.
High an low current is a reference to Amps not to volts.

So the high current fuse may blow but this will not affect the light in the switch as it is fed from the low current side which will most likely be on a different fuse.

If you remove both spades lugs from the demister circuit one should short to ground and the other should mate it's way all the way back to the switched side of the relay. to test this use a multimeter or a circuit test pen to follow the wires. if you need to find the relay leave the blowen fuse in as this should not affect the operation of the relay only the load (demister) on that relay, turn the demister switch from the on to the off position and listen to the relays this will give you the one you are after. the other way of doing this is to check at the fuse and put one of multimeter propes on the demister side of the fuse and the other one on the ground if this makes a circuit then you have a short on that wire (make sure you remove spade lugs from demister circuit) it's easy now replace the affected wire and this should fix the problem.
As to the problem with the clock it sounds like you drive to close to the Bermuda Triangle, I think that it is just a fault with the clock, but I might be wrong.
Hope this makes some sense
Adam