I've used a PocketPC jacket GPS and receiving signals is normally not a problem, unless you have a car with a windshield that has a [very thin] metallic layer, normally used as heat protection...
My new car [Peugeot 206], in which I haven't yet tried the GPS, has such a layer. Up around the rearview mirror the layer is missing and there is instead a dot pattern similar to that that is normally on the outside edge of car windows. It is intended to allow for mounting of things like toll-road tokens, and I think a GPS antenna mounted up there would work OK.
Will have to see how the sleeve GPS does - maybe the pattern is enough for it the signals to come through - perhaps only if it's mounted inconveniently high up by the rearview mirror...
/Michael
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/Michael