But it works on positions 1,2 & 3 that use the resistor pack, but doesn't
work on position 4 which is wired direct.

Much more likely to be either the switch, or a wiring/connector fault
surely?

Chris

The common resistor pack failure results in the blower fan NOT working
on positions 1 2 and 3  and  ONLY working on position 4.

I doubt if the fault being described is a resistor pack failure - or
more usually a thermal fuse in the resistor pack module.
Circuit showing arrangement of module and switch can be found at
http://www.amac.f2s.com/Focus1/

It is not unknown for the switch to mechanically fail. To remove the
switch you first have to remove the radio and in the bottom edge of the
hole left by the radio you will find the screws hold the fascia in
place. The bottom of the fascia should just clip on/off and all the
electrical controls will be on a connector.

Be aware that if you remove the power to the radio you will/may need a
security code to make it work again. If you don't have the code it can
be obtain from crack sites or ebay if you know the serial number of the
radio. The serial number is usually found on a label stuck to the top of
the radio :)