Check the fuse that supplies the heater has not corroded in the holder, had similar problem on my Webasto heater and that was the cause in my case.
I had a company in Poole sort mine out for me, if you search old forums, several years back now, you may find the post as I put up a recommendation for them...........but cannot remember their name, may have been Keto.
I was advised to run them for 10 minutes once a month summer and winter.
Good Luck