heated rear screen interferes with radio signal

[Woof25]

With the colder weather the rear screen heater comes on automatically below 10c I find this causes distortion of the radio signal with a sibilant hiss which dissapears when rear heater turned off any one have pics of how to access aeral etc and any suggested remedies to this problem other than an external aerial which our cars were not meant to have, although I suppose a shark fin style one would not be too noticeble if all else fails

I know there are many knowledgable members out there if anyone close to me I would be happy to visit or host a cream teas perhaps

Mike