Is this the antenna that came with the unit??
I think most Garmins use a powered antenna, (it has a little amplifier built into it).
Try a known good antenna.
Check you cable with a voltmeter. The end that plugs into the antenna should have 3 - 5 volts on it (center pin to outside shield) This is power for the internal amplifier.
As far as location, anywhere that has visibility of the sky should work. Microwaves pass through fiberglass like it wasn't even there, ON ONE CONDITION, the fiberglass is NOT painted with any type of paint that has a metalic based pigment, i.e. metal flake.
Waiter