Possible earth problem on 94(M) Fiesta

[Biggerz]

Can anyone suggest what could be causing this problem??

I have a 1994 M-reg Fiesta which runs perfectly ok apart from a weird thing with the electrics. When I am driving with the radio on and I switch on the rear window heater it makes the radio go fuzzy and crackly. When I turn it off again its a clear reception! This problme seems to be worse some days than others and some days it doesn't do it at all. Could this be anything to do with a faulty earth connection and if so which connection and how do I fix it?