The mod is very tough to do with the lighted visor (check the original post). It would seem to be simpler since you could just tap the vanity light, BUT the vanity light is only powered when the visor is in certain position. So you can not tap the vanity light power because this will cut power to your GPS when the mirror is flipped up. To add to the difficulty, the power to the visor is integrated into the bracket that the power screws into (you'll see what I'm talking about when you unscrew the visor from its mount). I trashed my original lighted visor trying to figure out what was going on with the power to the vanity lights. Best bet is to get an unlighted visor from salvage/ebay and just run a new circuit to the insider of the visor. The visor post on the unlighted visor is *much* shorter and ends about halfway inside the visor so it's very easy to run the wires to the GPS once you pop the plastic end cap off the visor post.
The other tricky part of the mod was getting a connector that would fit the back of the unit and not stick out too far. I had to cannabalize a mini-usb adapter and bend it a 90-degree angle to get it to mount more closely to the back of the Garmin.
BUT, one thing to consider if you are thinking about doing this mod now: The newer Garmins are a bit slimmer so you may not have to worry about modding the USB power plug. And I believe the new 5" screen models pretty much fill up the hole left by the mirror (I borrowed a friends to do a quick test fit). I might upgrade to a 5" model in the next few weeks since my original 4.3" model just died--battery went bad I think. The boot time on the new models is a lot shorter, and for the same price point I got my 285W a couple years ago you get some new features too, like voice command. If I do an upgrade, I will post up some new pics.
To answer another question, no I don't get any droop in the visor from the added weight, but my visor was tight and snapped firmly into position to start with.
