I write as a Sierra 4x4 driver of some six years' standing, and share WB's enthusiasm for the breed. However, I have been meaning to write for over five of these years, ever since he first started to complain of the washer water freezing up.
I had the same problem, but solved it completely by adding about 20 per cent methylated spirits to the water. The added advantages are that the screen is rendered squeaky clean (literally!) and the aroma which permeates the cab is quite pleasant.
Tom Candlish, Burnham, Bucks.