where are the heater plugs on my van???

I have a 1991 fiat ducato motorhome. 2.5 turbo engine. I can not find the heater plugs (if any) I do have an electrical unit on top of the manifold which has fuel running through it then on to the injectors. I have been told 1. it doesn't have heater plugs just a fuel heater in line. 2 the plugs are down the back of the block ( I have looked with mirrors can't find any) 3 they are under the rocker cover. No they arn't Ive looked there as well. I don't have a problem starting but I would like to know where to look if and when I do get a problem. Two fiat garages can't find them either but both say the van should have them. Is there any body who can give me a difinitive answer as I am rapidly going bald pulling my hair out.