I've used them on the front dash for 2 years, no problems whatsoever. This is in 3 cars. Just put them right up next to the glass. Use velcro to secure it in place.
I used double sided tape and installed it just behind the speedometer cluster on the flat spot of the dash. Right at the base of the glass. Recently drove from Virginia to Florida on I-95 with no drops, even in really heavy rain.
