VW Sharan auxiliary heater malfunction

Hi!

I'm having a problem with the auxiliary heater in my 1999 Sharan, which has an engine power of 81 kW.
She has been refusing to provide her services for about 4 weeks!
:( ...and that at these temperatures!!!
First, I checked the fuse (number 24). OK!
As a second step, after the wiring (plugs, cables, connectors). OK!
Next, I want to check the spark plug!
Or does anyone have experience and can offer me some advice?

Martin.