Mk7 keep blowing heater/blower fuse

[sinvar]

Hi!
As the title say, it keep blowing the fuse and I can´t see any pattern for when it blows. I have understood that the resistor is a typical problem, but I can´t understand how that can be my problem? It can last for days or just a few minutes.
Can´t find any other subjects that cover just the same problem, but is there any typical things I should check?