The underlying problem is probably a blocked pollen filter. The airflow
through the pollen filter cools the resistor pack. When the filter gets
blocked the resistor pack overheats.

First, it's nothing to do with the Air Conditioning.

The circuit for the blower is relatively simple. On the fast setting the
fan is straight across the 12V from the battery. On the slower setting a
set of resistors lowers this voltage. A thermal fuse protects these
resistors (stopping them catching fire) and it is this thermal fuse that
goes.

Circuit and photographs at:
http://www.amac.f2s.com/Focus1/

As the resistors are getting too hot possible causes could be:

i) There is some obstruction in the air flow and/or the pollen filter is
blocked.

2) Water has entered, via the pollen filter/air intake, and the motor
shaft has become rusty and the motor is binding on its bearings.

3) Duff fan motor.

The pollen/cabin filter cover on the early Focus is on the passenger
side at the bottom of the windscreen (the seal to the windscreen is part
of the cover). To save costs, on some models Ford didn't actually fit a
filter element but the holder is still there as it is also the air
intake to the inside of the car.  If no filter is fitted check for any
rubbish that may have entered the air intake.

Be aware that that is not uncommon for an incorrectly fitted cover to
allow (rain) water to fill up the passenger foot well.  When I removed
the cover from my Focus pollen filter one of the plastic clips broke :(
I also had to use some rubber sealer on the bit that attaches to the
windscreen.

For motor problems you will have to remove the fan assembly. If the
problem is a rusty shaft, and if you can get to the bearings, a few
drops of oil may be all that is required. Don't use something like WD40!
Use a decent thicker oil engine oil and apply one or two drops, using
the dipstick,  to each of the bearings. Make sure that the fan spins
fairly freely before re-installing.

Hey thanks guys thats a GREAT HELP :)  ! At least now I might get to
resolve the problem. Will keep you posted but thanks. I live in
Scotland and we need the heater over here  :lol