What kind of power do you have available in rearview mirror and the wire that is on the windshield to the compass (I think)? It is suitable for hard wiring a GPS unit or Radar detector?
Garmin GPS units, as well as any other brands that use a USB connection to a computer for updating maps and firmware, or for charging the internal battery, require a voltage between 4.75 V and 5.25 V. This is the USB specification. Computer power busses also supply 12 volts DC, which devices such as cooling fans use, but not at the USB port. I'm quite sure that as MalibuKen suggested, you would fry the electronics of most GPS units by applying 12 Volts to the power port.
This is not true of radar detectors, by the way. They use 12 volts DC and do not require the use of a separate power supply (such as the cigarette lighter power adapter on your GPS) and you can dispense with the lighter adapter for direct wiring.
That said, I used the lighter adapter with my Valentine One radar detector, and hard wired it using a female lighter adapter because it was convenient to do so.
I fail to see how using the power from the mirror, or the sunroof if your car has one, would be ideal even if the voltage were correct. The windshield on the Malibu has an extreme rake, which effectively reduces your field of vision. Mounting a GPS on the windshield, or even on the top of the dashboard, would put it out of convenient reach of the driver as well and would in most cases reduce your field of vision if mounted in either location. Mounted out of your field of vision on the windshield or on top of the dahs puts a GPS too far away to effectively use the touch screen, as well as making it more difficult to see. At night, the lighting on the GPS touch screen would quite likely be very distracting.
There are any number of after market mounts that can be used in various locations on the front of the dash near the radio. I used a vent mount and mounted my Garmin using it. I was able to run the power cord from the passenger side fuse block using a Littelfuse Add-A-Circuit and a female lighter adapter socket, up behind the dash and under the cubby tray on the top of the dash which is easily removed by removing two hex screws under the rubber nubbies that support the closed lid. This places the GPS in an ideal position allowing the easy use of the touch screen. It is as convenient to use as the radio or ventilation controls on the dash. In my installation there is only a very short length of power cord--around 5"--which is nearly invisible because it is behind the GPS unit. Since the circuit I used at the fuse block is switched by the ignition switch, the unit comes on and shuts off whenever the ignition switch is in the "on" or "off" position.
Recent Garmin GPS units have a traffic monitor function that requires an antenna which is either within the cigarette lighter adapter at the end of the power cord, or inline on the power cord itself. It requires a power supply, although I am not certain whether the voltage is critical.
Most importantly, if you cut the power cord you may well void your warranty on your GPS. I wouldn't try it.