Rear washer jet freezing

[pg_mondeo]

Hi all
Just wondering if anyone else has had the rear washer jet freezing up?

I live on the other side of the Blue Mountains and on the (all to regular) frosty mornings we get here the back window of my MA XR5 is a sheet of ice.

I had read somewhere that the washer jets were supposed to be heated but this may only be the front although these have also frozen on a couple of occaisions

Any advice on what I could put in the washer water to prevent it from freezing that wouldn't be a risk to the paintwork?

Thanks!