The windscreen in the 172/182 is a b*tch for this, my saving grace is that I use a PDA which means my GPS unit sits on the parcel shelf and gets full signal
One thing worth doing is leaving the snooper out in the garden for an hour or so (only need to do this once), this way it will build a cache of all the satellites in view and will provide faster & better fixes.
Same thing goes for when you get in the car, make sure you have a good satellite fix before you move off, GPS doesn't like fixing onto satellites when you are moving.