Oh dear , err well i guess the best way to explain this , is it's a design issue , as far as i know on the x230 base , though i can't say , limited only to this model .
I replaced my fuse box about two , three years ago with exactly the same problem . I brought my Hymer back in 2017 , and though never noticed this during the pre purchase check , found it a couple of weeks later . It seems to be a common issue on the 230 base , caused by excessive use of the heater blower motor used in position 4 . It was something to do with excessive current causing enough heat to quite literally melt the fuse block . I never went too deep into this , as the way i understood it , it was a design issue and basically unrepairable . The best advice i can give , is do not use position 4 , of the blower motor for any great length of time , the fuse holder is basically not designed for this kind of abuse . There are fuse holders that can handle this current , but on that note i can't answer if the heater can or not . I also have no idea before anyone asks , if this is restricted to just the 230 , or whether any other models are affected . What i can say is , it was well documented on the net , with others reporting a similar problem .
Fuse blocks are difficult to source for these vehicles , and personally i searched for three years , until i eventually located one in a breakers yard in Lithuania . I did have a photograph of my fuse box , but seem to have lost it . I think it's on my other computer which had a brain storm , and died on me last year . I'm still trying to recover the pictures and music even now . Examination revealed the remains of two melted fuses , with the actual box badly burnt both sides of position 13 , incidentally a 30 amp blade position . I also found a bypass external fuse holder while working on the electrical system around the same time , ironically connected with 10 amp household connector blocks , when i replaced the original box . Disconnecting this , i reinstalled the heater wires back to their original positions back on the fuse block .
My own vehicle is a year 2000 , Hymer 584 , based on an x230 , 2.8 tdi chassis . It's been off the road for three years now , and due to return next year , after some pretty extensive work . Best advice , use the central heating , but don't use the cab heater for extended periods on position 4 .