heater issues

hello everyone, new to the site

I own a 2012 plate skoda superb 2.0 tdi S 6 speed manual

owned a fabia vRs in the past then went to a bmw...now back again

iv owned my 2012 skoda superb since april 2017 and its been a great family car.  iv never needed to "research" any issues unlike my last bmw where I was regularly on the forums lol

so my first issue which I thought was sorted has come back

when I got the car , I noticed the heaters only worked  on the 4th setting. after some research I bought a replace heater resist off ebay and it worked for about 6-8 months

then same again...this time I bought a pack of 4 resistors and used the same casing and replaced the resistor itself . 

again worked ok for about 2 months.

not its winter and iv got the same issue again/

can someone advise me as to what is causing the heater resistor to fry and how to fix it please 

many thanks