Focus mk1 1.6 petrol hatchback 

I've replaced the heater resistor twice (febi) within a month at the time I also replaced the heater fan but the resistor has failed again only works on setting 3/4

Any ideas why this is the case?

I've ordered another one but the last thing I want is for my third resistor to also fail

When the first original resistor failed the car filled with smoke, not sure whether this could have damaged the wiring hence why the resistors are always failing?

I also read elsewhere online the following...

The spades on the resistor pack get hot and damage the terminals in the plug. So, when changing the resistors, you have to cut the plug off and solder on a short length of new loom and modified plug.

If you don't, then you will be changing resistors on a regular basis.

Has anyone experienced the same and can shed any light to fix this issue 

3 hours ago, ash29109 said:

Any ideas why this is the case?

The blower motor is stiff, possibly the bearings are seizing up. The motor is therefore drawing excess current causing the resistor pack to burn out.

Try getting putting spray 3-in-1 oil on the blower fan bearings.

3 minutes ago, unofix said:

The blower motor is stiff, possibly the bearings are seizing up. The motor is therefore drawing excess current causing the resistor pack to burn out.

Try getting putting spray 3-in-1 oil on the blower fan bearings.

It's a brand new blower motor which I replaced at the time when I fitted the second new resistor 

Then I would suggest the resistor packs are of poor quality.

Fitted a new resistor without fixing it in place, the green element of the resistor became very hot, is that normal?

However the fan only works on setting 2 and 4

I'm thinking whether it could be the heater control at fault?

5 minutes ago, ash29109 said:

Fitted a new resistor without fixing it in place, the green element of the resistor became very hot, is that normal?

Yes.  It needs to be fitted in the airflow of the fan to keep it cool.

A resistor works by literally wasting energy, turning it into heat instead of fan rotations.  That heat needs to go somewhere.

I've replaced the resistor, new fan, new pollen filter, what else could it be? Only thing that comes to mind is that it could be the heater control?

Heater control switch was the culprit, some of the terminals within the switch had melted!