This was posted before. Thought I'll post it up again, since there is a rise in car break-in and stolen GPS. Unfortunately, this only works for the Mazda6. I hope the new 2008 Mazda6 will have the same storage compartment, or something similar.
Advantages:
1) Easily hides the GPS unit, without leaving any visible signs, such as suction cup stain etc.
2) Charges the GPS unit without the sight of messy wires.
3) Looks stock (kind'a... :P)
This setup requires you to run a cigarette lighter extension (CigExt) cable into the centre storage compartment, to charge the GPS unit. I'm tapping the power from the cigarette lighter itself. I'm sure some other members can tap it directly from the battery or something.
You need something like this:

Make sure the plug is small enough so that you can still close the ashtray lid.
__________________________________________________ ___________
1) First remove the headunit:
Link: Remove Headunit
or
Link2: Remove Headunit from iPod install
2) After removing the Storage Box: The Storage Box and the LCD screen comes in one piece.
3) Drill a hole behind the Storage Box:
4) Run the wire for the CigExt through. You might need to slice the wire to get it through the small hole, and then solder it back together again. Tape it in place under the storage box (see pic)
5) Run the wire in between the vents, and then along the sides behind the HU. There’s a small hole that allow the wire to go through into the ashtray compartment.
6) Put the gear-shifter cover & ashtray compartment back on. While doing that, slide the CigExt plug through the back of the ashtray lid.
7) Tape the CigExt output in place. Plug in the GPS charger and tie the wires neatly.
8) I then cut some black cardboards into shape and taped them against the wires to hide them.
9) I hold the GPS in place with some foam tapes (Hard - high density ones) I got from Bunnings (weather-shield dept). Top and Bottom. Because the charger plugs into the bottom of the GPS unit, I have to elevate the unit with 3 layers of foam padding. I did this in November 2006, and today the GPS still fits securely. Has never rattled or slipped.
10) To hide the GPS, simply slide it out of the foam pads, put it in horizontal position, and close the lid.