I had one of my 6ALs mounted directly behind the headlight area on my old '84 Sunbird (North American car, not sure of Australian equivilant), and only ever had a problem with moisture one time, when it was snowing, but really heavy and wet snow. Enough of this heavy wet snow came in through the grill and past the headlights to cause my car to shut off after driving in this for about 40 minutes or so, at highway speeds.
After scratching my head for a bit and making a call to my uncle, to get his input, mind you this was at about 11:30 at night, on a poorly lighted highway, I finally found the issue, and was simply that the magnetic trigger input was shorted from the heavy snow. Wiped it off with a slide of my thumb and was on my way again. The box worked fine for a long time after that, even on the next car, where I mounted it in a different spot.
The MSD 6AL is potted, and if you can have any of teh electronics directly effected by moisture you're doing something very wrong. lol