I have seen burned opel astra diesel because of a lack of fuses! The shorted wires under the hood weren't fused at all.
Normally the washer fluid reservoir should be fused, but it seems that some models in the past had some problems with that. I guess that if you take of the connector at the reservoir side of the heater you will be save (since the system will no longer use any power than).
with a minor 600+k