General heater rheostat needed but can't find anywhere!

[hmgvs]

I have a hymer campervan on a Fiat Ducatto 2.5 TD 1993.
I need a heater rheostat but as it's an import vehicle Fiat are unable to help as are the uk Hymer dealers.
Anyone have any suggestions?
may be able to repair it if someone could give me a pointer!