Hi Heno,
Yes it does need to see the sky but there are parameters. Your device stuck under your roof is probably picking up 'bounced' or reflected signals through the windscreen and rear windows.
The GPS signals will travel through anything non ferrous, i.e wood, plastic, cloth, but not metal.
Inside the cabin of a vehicle it may receive signals through the windows but not the roof (unless it is a soft top). I have had mine in a glove box and it still works by receiving the signals through the windscreen.
If placed on the crash bar within the rear bumper (normally plastic these days), it should get a good accurate fix.
Every vehicle is different and in most cases you have to experiment.
In addition, you have to add in the 'urban factor'. Heavily built up areas will only let the satellites vertically above to be received whereas in a more open area more satellites will be picked up that are oblique to you and a more accurate fix given.
Hope this helps.
Regards
Peter
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